Assalamualaikum ustaz. Is the prayer of a man valid if he prays while wearing clothing that has a small hole the size of a needle? The hole on his clothing is under his navel. Hope for an explanation.
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.
We start with the statement of Allah SWT:
يَا بَنِي ءَادَمَ خُذُوا زِينَتَكُمْ عِنْدَ كُلِّ مَسْجِدٍ
“O children of Adam, take your adornment at every masjid,” 
Ibn Kathir explained, in the meaning of this verse, it is sunnah to wear neat and appropriate clothing especially on Fridays and during Eid. It is also encouraged to wear perfumes and brush one’s teeth for both are included as adorning oneself and a form of perfection. 
The basis of it is to cover the aurah which is the valid condition of prayer. If the aurah is exposed during prayer then it is obligated to immediately cover it back. If a person neglected and just left his aurah exposed not promptly covering it, then his prayer is invalidated and he has to start again. However, if he managed to cover his exposed aurah immediately, then his prayer isn’t invalidated and he can continue praying. 
Imam al-Syirazi said: “Hence, if any of the aurah is exposed, when he can cover it, then the prayer is invalid.” Imam al-Nawawi then explained saying: “His prayer is invalid whether the aurah exposed is a lot or a little…” 
Imam al-Nawawi also emphasized saying: “Covering aurah is a valid condition of prayer. If any of the aurah is exposed, then the prayer is invalid. Regardless of whether it is a lot or a little. Man or woman. Prayer performed alone or in congregation. Whether it is obligatory or sunnah or funeral prayer or tawaf or tilawah or prostration or prostration of gratitude.” 
Aurah for man in prayer is the part between the navel to the knees. Thus, it is obligatory for a man to cover this part with clean clothing free from najis when he is praying. If he let his aurah get exposed purposely, then the prayer is invalid.
According to Imam al-Nawawi, it is obligatory to cover the aurah with clothing that doesn’t let the skin color be visible and it cannot be covered by thin clothing. It is sufficient to cover the aurah with any type of clothing, leather, paper or woven leaves as long as the skin color isn’t visible. 
Thus, covering the aurah properly is included as a valid condition of prayer. This is based on the admonishment of the Prophet PBUH to Jarhad when his thigh was exposed during prayer. The Prophet PBUH said:
غَطِّ فَخِذَكَ فَإِنَّهَا مِنْ الْعَوْرَةِ
“Cover your thigh, for indeed it is ‘Awrah.’” 
Coming back to the above question, it is clear as we’ve stated that covering the aurah is one of the valid conditions of prayer. If a person prays without covering his aurah, then his prayer is invalid and he has to pray again. Aurah for a man is the area between his navel and knees, while for a woman, her whole body except her face and palms according to madhhab Syafie. Their opinion is based on the hadith of Aisyah R.Anha:
لَا يَقْبَلُ اللَّهُ صَلَاةَ حَائِضٍ إِلَّا بِخِمَارٍ
“The Salat (prayer) of a woman, who has reached puberty, is not accepted unless she is wearing a Khimar.” 
Al-Tirmizi said: “This hadith from Aisyah is a hasan hadith and is practised (nad used as argument) by scholars. Al-Nawawi said, “The word “ha’id” in this hadith means a woman who has reached puberty (menstruated).” And this is also a valid condition for men. 
The extent of covering the aurah as stated by Imam al-Syairazi in al-Muhazzab (125/1), “and the obligated covering of the aurah is so that it is not similar to one’s skin color.” Syeikh Zainuddin al-Malibari said: “(covering the aurah) with something that doesn’t make the skin color visible in a speech ceremony.” 
The extent that it can be seen in a speech ceremony is discussed further by Syeikh Abu Bakar al-Dimyati as: “The situation where a person who is watching the person who is praying has to approach until he is very close to be able to see the skin color, then this doesn’t affect the prayer (means the aurah is not exposed).” 
The meaning from the above statement is that the valid condition of prayer is it is sufficient for a person who is talking to his friend to not be able to see the aurah of his friend.
Thus, in our opinion, the hole is the size of a needle hole in clothing where if a person prays while wearing it, on the aurah area, then it doesn’t invalidate the prayer.