What is the ruling on a person who does not completely sit during the Tashhahud? What if the person is sitting but is not completely at ease and low in their sitting position due to a fear of relaxing too much and consequently passing wind during the prayer? This happens to me throughout prayer, and I often feel that I have not met the requirement of sitting during the prayer since my backside does not completely rest on my legs/heels sometimes. If a person does not have a physical excuse for not sitting completely, does it affect the validity of their prayer? Does not completely sitting on one's legs/heels during the Tashhahud and in between prostrations affect the validity of the prayer? What are the specific obligations regarding sitting during the Tashhahud? Due to this uncomfortable feeling when trying to sit completely during the Tashhahud (possibly because I am about to pass gas), I tend to move my backside up and down in order to avoid passing gas. In this situation, and even if I am not seated properly, can I continue to recite the Tashhahud? May Allah reward you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

If you mean by not sitting completely that your backside does not fully rest on your legs/heels, but it settles in a certain position while reciting Tashahhud, this is sufficient. What counts is to feel Tuma’neenah (tranquility) during Tashahhud.

Ibn Qaasim said,

"Tuma’neenah means tranquility for the duration of reciting the obligatory Thikr (expressions of remembrance of Allah). A number of scholars underlined that the criterion of observing Tuma’neenah is that one feels tranquil and his body parts settle. It seems that the author did not mention that it is obligatory to tranquil for the duration of reciting the obligatory Thikr because it is known from what was said before. Rather he defined Tuma’neenah using the linguistic terminology instead of the Islamic terminology which is to observe tranquility for a period of time enough to recite the obligatory Thikr; as this is an obligation, and other than that is not permissible. This was cited by Al-Wazeer and others on the authority of most scholars." [Al-Haashiyah Ala Ar-Rawdh Al-Murbiʻ]

Sitting while reciting Tashahhud is sufficient in any given manner; An-Nawawi said after a detailed description of the manner of sitting in this position:

"Our scholars held that it is not incumbent to sit down for the Tashahhud in a specific manner; any given manner is sufficient in terms of clearing liability from the obligation in this regard. Whatever manner he sits in would be sufficient for him, whether it is Tawarruk (in which one's buttocks are on the ground while the left foot is laid down and the right foot is held upright, and both are held out to the right), or Iftiraash (i.e. bending one's left leg underneath himself, and sitting on it while erecting his right foot with its toes pointing towards the Qiblah [prayer direction]), or with the legs stretched out or both knees bent upwards or one of them or other than that. However, the Sunnah is to sit in the Tawarruk position when reciting the final Tashahhud and in the Iftiraash position in other sittings in prayer." [Al-Majmooʻ]

Assuming that your backside is not settled while reciting the Tashahhud for fear of passing wind, your prayer is valid because maintaining your ablution is given priority over sitting properly for the Tashahhud as one of the pillars of prayer. Shaykh Al-Kharashi said, "It is an obligation to perform the obligatory prayer while standing unless standing involves hardship or fear of harm or passing gas if he stands up and it can be avoided by sitting, so he prays while sitting. This scholarly view was underlined by Ibn ʻAbd Al-Hakam, since observing the condition that must exist for the duration of the act of worship is given priority over observing one of its obligatory pillars in general." [Sharh Mukhtasar Khaleel]

The Shaafiʻi book Al-Iqnaaʻ reads, "If the person suffers from urine incontinence such that if he stands he discharges urine and if he sits he does not, then he should perform the prayer while sitting according to the soundest scholarly view in this regard, and he is not required to repeat the prayer."

Lastly, it should be noted that the first Tashahhud is an act of the Sunnah according to the majority of the scholars. The last Tashahhud, on the other hand, is one of the pillars of the prayer. If it is left out, the prayer is invalid according to the Hanbalis and Shaafi'is, contrary to other scholars.

Allah knows best.