Assalamualaikum ustaz. Is it possible for the devil to shift into a human?
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Allah SWT states:
يَا بَنِي آدَمَ لَا يَفْتِنَنَّكُمُ الشَّيْطَانُ كَمَا أَخْرَجَ أَبَوَيْكُم مِّنَ الْجَنَّةِ يَنزِعُ عَنْهُمَا لِبَاسَهُمَا لِيُرِيَهُمَا سَوْآتِهِمَا ۗ
“O children of Adam, let not Satan tempt you as he removed your parents from Paradise, stripping them of their clothing to show them their private parts.” 
The above verse is a reminder for all mankind to not be tempted by the lies and deception of the devil. They beautify sins in your eyes just as they fooled your ancestors the Prophet Adam and Hawa’ (Eve). Due to the sin, both of them are exiled from paradise and humiliated when their clothing is stripped of which Allah SWT protected (their awrah and private parts) leaving them naked. 
The devil is the number one enemy of believers. They will strive to lead the children of Adam astray by bringing them closer to sin by beatifying them (in the eyes of people).
Evidences of the Iblees and Shaytan Shifting into Human Form
There is a story of when a devil shifted into a person and ask for zakat from Abu Hurairah RA who was entrusted to take care of the zakat, he said:
وَكَّلَنِي رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ بِحِفْظِ زَكَاةِ رَمَضَانَ، فَأَتَانِي آتٍ فَجَعَلَ يَحْثُو مِنَ الطَّعَامِ، فَأَخَذْتُهُ فَقُلْتُ: لَأَرْفَعَنَّكَ إِلَى رَسُولِ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، فَقَصَّ الحَدِيثَ، فَقَالَ: إِذَا أَوَيْتَ إِلَى فِرَاشِكَ فَاقْرَأْ آيَةَ الكُرْسِيِّ: لَنْ يَزَالَ مَعَكَ مِنَ اللَّهِ حَافِظٌ وَلاَ يَقْرَبُكَ شَيْطَانٌ حَتَّى تُصْبِحَ. وَقَالَ النَّبِيُّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: صَدَقَكَ وَهُوَ كَذُوبٌ، ذَاكَ شَيْطَانٌ
“Allah’s Messenger (ﷺ) ordered me to guard the Zakat revenue of Ramadan. Then somebody came to me and started stealing from the foodstuff. I caught him and said, “I will take you to Allah’s Messenger (ﷺ)!” Then Abu Huraira described the whole narration and said: That person said (to me), “(Please don’t take me to Allah’s Messenger (ﷺ) and I will tell you a few words by which Allah will benefit you.) When you go to your bed, recite Ayat-al-Kursi, (2.255) for then there will be a guard from Allah who will protect you all night long, and Satan will not be able to come near you till dawn.” (When the Prophet (ﷺ) heard the story) he said (to me), “He (who came to you at night) told you the truth although he is a liar; and it was Satan.” 
There is a narration of how an iblees shifted into a man and promised good things as narrated from Ibn Abbas with a disputable sanad from Ali bin Abi Talhah. Ibn Abbas R.Anhuma said:
جَاءَ إِبْلِيسُ يَوْمَ بَدْرٍ فِي جُنْدٍ مِنَ الشَّيَاطِينِ مَعَهُ رَأَيْتُهُ فِي صُورَةِ رَجُلٍ مِنْ بَنِي مُدْلِجٍ فِي صُورَةِ سُرَاقَةَ بْنِ مَالِكِ بْنِ جُعْشُمٍ ، فَقَالَ الشَّيْطَانُ لِلْمُشْرِكِينَ : لاَ غَالِبَ لَكُمُ الْيَوْمَ مِنَ النَّاسِ وَإِنِّي جَارٌ لَكُمْ ، فَلَمَّا اصْطَفَّ النَّاسُ ، أَخَذَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَبْضَةً مِنَ التُّرَابِ ، فَرَمَى بِهَا فِي وُجُوهِ الْمُشْرِكِينَ ، فَوَلَّوْا مُدْبِرِينَ . وَأَقْبَلَ جِبْرِيلُ إِلَى إِبْلِيسَ ، فَلَمَّا رَآهُ ، وَكَانَتْ يَدُهُ فِي يَدِ رَجُلٍ مِنَ الْمُشْرِكِينَ ، انْتَزَعَ إِبْلِيسُ يَدَهُ ، فَوَلَّى مُدْبِرًا هُوَ وَشِيعَتُهُ ، فَقَالَ الرَّجُلُ : يَا سُرَاقَةُ تَزْعُمُ أَنَّكَ لَنَا جَارٌ ؟ قَالَ : (إِنِّي أَرَى مَا لاَ تَرَوْنَ إِنِّي أَخَافُ اللَّهَ وَاللَّهُ شَدِيدُ الْعِقَابِ) وَذَلِكَ حِينَ رَأَى الْمَلاَئِكَةَ .
“During the battle of Badar, Iblees came in rows of army and I saw him in the form of a man from Bani Mudlij a man named Suraqah ibn Malik ibn Jus’um. He said to the idolaters: “None can defeat you on this day and I am your protector. When people began to arrange themselves in rows, Rasullullah PBUH grasp a handful of dirt and throw it towards the idolaters and they ran in chaos. Then, Gabriel moves towards iblees. At the time, the iblees was holding the hand of an idolator, upon seeing Gabriel, he releases the hand of the musyrik and iblees together with all the devils accompanying him ran away. The idolator (musyrik man) said: “O Suraqah, weren’t you the one who told us that you are our protector?” Iblees answered: “Indeed, I see what you don’t see and I fear Allah. And Allah’s punishment is grave.” This is the time when iblees sees the angels.”
This story is narrated by Imam al-Tobari in his commentary and Ibn Kathir in the commentary os durah al-Anfal.  al-Tabrani also narrated in al-Mu’jam al-Kabir from Rifa’ah bin Rafi’ al-Ansariy as stated in narration by Ibn Abbas, however, its sanad is dhaif because of Abd al-Aziz bin Imran, who is dhaif. Al-Haithami has evaluated it dhaif because of him in Majma’ al-Zawaid. 
Is it possible for a devil to shift into human form?
Syeikh Muhammad al-Syibli stated that it is undeniable the jinn and shaytan are capable to shapeshift, regardless of whether it is into the form of a human or animal. Just like when it shifted into Suraqah bin Malik bin Ja’syam and came to the Quraisy idolaters when they are headed for the battle of Badar. 
Syeikh Sulaiman al-Asyqar stated that sometimes the devil will come to people not only in whispers of doubt but also in human form. We can also hear its voice without seeing its body. It can also shapeshift into weird or scary forms. 
Al-Qadhi Abu Ya’la said that the devil does not have the ability to shapeshift. However, Allah SWT taught several words and these several words and actions can change their form into something different. 
Although the devil and jinn can change their forms or shapeshift into something else, however, one should remember that such action can never harm us. Harm or benefit can only come from Allah SWT.
This is further strengthened through the words of the Prophet PBUH narrated from Jabir bin Abdullah R.Anhuma, Rasullullah PBUH said:
لا عَدْوَى وَلاَ صَفَرَ وَلَا غُوْلَ
“There is no infection, no serpent in a hungry belly, and no ghoul.” (It was used of a creature which was a type of jinni or devil which was believed to appear to people in various forms and lead them astray in the desert and cause their destruction).” 
Syeikh Muhammad bin Soleh al-Uthaiin in his commentary stated: Ghoul is a type of devil that usually leads people who are travelling in the desert or valleys astray. In this hadith, Rasullullah PBUH rejects the influence of ghoul and not its existence. Devils who like to disturb humans such as the ghoul do exists, however when a person’s tawakkul towards Allah SWT is strong and ignores its presence, the devil cannot harm and prevent him from travelling where he intends to go. 
Al-Khattabi when commenting on this hadith said that this hadith doesn’t deny their existence and shapeshifting ability, however, this hadith denies their ability to deviate and harm humans. Don’t be afraid of them, for they are incapable of doing anything without Allah SWT’s permission. He also stated that a Ghilan is a sorcerer from among the jinn. 
Thus, we state that the devil or iblees can shapeshift into various forms and faces which include humans just like in the incident of a human asking for zakat and teaching al-Kursi and the devil resembling Suraqah bin Malik during the battle of Badar.
May Allah SWT grants is understanding in this religion and protect us from the temptations and disturbances of the devil and their progeny. Amin. We end with the supplication:
نَعُوذُ بِكَلِمَاتِ اللَّهِ التَّامَّاتِ مِنْ غَضَبِهِ وَعِقَابِهِ وَشَرِّ عِبَادِهِ وَمِنْ هَمَزَاتِ الشَّيَاطِينِ وَأَنْ يَحْضُرُونِ
“I seek refuge in the Perfect Words of Allah from His anger and His punishment, from the evil of His slaves, and from the taunts of devils and from their presence.”
 Surah Al-‘Araf : 27
 See al-Tafsir al-Muyassar, pg. 153
 Narrated by al-Bukhari (5010)
 See Tafsir al-Tobari, 13/7
 See Tafsir Ibn Kathir, 2/317
 See al-Mu’jam al-Kabir, 5/47
 See Majma’ al-Zawaid, 6/82
 See Aakam Al-Marjan Fi Gharaib Al-Akhbar wa Ahkam Al-Jann, pg 21
 See Silsilah Al-‘Akidah : Alam Al-Shayatin Wa Al-Jinn, pg 91
 See Aakam Al-Marjan Fi Gharaib Al-Akhbar Wa Ahkam Al-Jann, pg 21-22
 Narrated by Muslim (2222),Ibnu Hibban in his Sahih (6128), Ahmad (15103) in Musnad. Syeikh Syu’aib Al-Arnaut stated the sanad of this hadith is sahih implementing the conditions set by Imam Muslim.
 See al-Qaul al-Mufid, 1/569
 See Ma’alim Al-Sunan Fi Syarh Al-Sunan, 4/234