Assalamualaikum ustaz. Is it permissible for a person who is performing Qasar to follow an imam who is performing Tamam? For example, I want to perform Qasar Zohor. Is it permissible for me to follow an imam who is performing tamam Zohor in a situation where I know that the imam is performing tamam prayer?
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 Last Day.
Allah SWT gives facilitation (rukhsah) to people who are travelling to perform their prayer in jamak and qasar. Allah SWT states in the Quran:
وَإِذَا ضَرَبْتُمْ فِي الْأَرْضِ فَلَيْسَ عَلَيْكُمْ جُنَاحٌ أَن تَقْصُرُوا مِنَ الصَّلَاةِ
“And when you travel throughout the land, there is no blame upon you for shortening the prayer,” 
According to Imam al-Nawawi, this verse relates the ruling of qasar with al-dorb `ala al-ardh (travelling). Hence, if a person lives in a city, then it is permissible for him to perform qasar when he leaves the last building before the outskirts of the city. 
In another hadith, from Ya’la bin Umayyah, he said:
قُلْتُ لِعُمَرَ بْنِ الْخَطَّابِ : ( لَيْسَ عَلَيْكُمْ جُنَاحٌ أَنْ تَقْصُرُوا مِنَ الصَّلَاةِ إِنْ خِفْتُمْ أَنْ يَفْتِنَكُمُ الَّذِينَ كَفَرُوا ) (النساء: 101) ، فَقَدْ أَمِنَ النَّاسُ ؟ فَقَالَ: عَجِبْتُ مِمَّا عَجِبْتَ مِنْهُ ، فَسَأَلْتُ رَسُولَ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ عَنْ ذَلِكَ، فَقَالَ: ( صَدَقَةٌ تَصَدَّقَ اللهُ بِهَا عَلَيْكُمْ ؛ فَاقْبَلُوا صَدَقَتَهُ )
“I said to ‘Umar bin Al-Khattab: ‘There is no sin on you if you shorten salah and if you fear that the disbelievers may put you in trial (attack you). But now the people are safe.’ ‘Umar said: ‘I wondered the same thing, so I asked the Messenger of Allah (ﷺ) about that and he said: This is a favor from Allah (SWT) to you, so accept His favor.” 
According to Imam al-Zarkasyi, performing qasar prayer (during travels) is prioritized over praying in full (tamam/complete). 
In the above issue, it is impermissible to perform qasar prayer following behind a tamam imam (the complete four rakaat) for it is not in line with the valid condition of qasar prayer. 
This is based on a narration from Imam Ahmad from Ibn Abbas RA, when he was asked: “Why is it a traveller perform two rakaat prayer when he is alone and four rakaat when he follows an imam who is a resident there?” Ibn Abbas replied: “Such is the sunnah.” 
Syeikh Salim bin Suamri al-Hadrami among others state: “He shouldn’t be the makmum of a complete prayer for parts of his prayer.” 
Some scholars cited this ruling if the ijma’. Imam Ibn al-Munzir said: “And they (scholars) have agreed that when a resident is the makmum of a traveller and when the imam has completed his two rakaat prayer, the makmum has who is a resident has to complete his prayer.” 
Hence, when the qasar prayer fails to fulfil its valid conditions, then the prayer is invalid if he doesn’t complete the four rakaat prayer. It is obligatory for him to replace the prayer through tamam when he is no longer travelling and it is permissible for him to perform qasar as long as he is on his travels.