What is the meaning of ṭumaʾnīnah (tranquility/stillness) in bowing, and what is its feeling, and how is it achieved? And what should a congregant do if he repeatedly omits a pillar such as "ṭumaʾnīnah", or "straightening the spine in bowing", or "following the imam in prostration" due to the imam's speed? And is he excused for that due to illness, or must he repeat it after the imam's taslīm?

Tranquility (al-Tuma'ninah) is a pillar of prayer, meaning the stillness of the limbs from movement, even if for a moment, or for the duration of the obligatory remembrance (dhikr). If the limbs become still after bowing for Rukoo', then tranquility has been achieved. However, if the bowing is without stillness, the prayer is invalid.

Tranquility is not a spiritual act like devotion (khushoo'), but rather the stillness of the limbs. If the Imam rushes in a way that violates tranquility, then prayer behind him is invalid. However, if his speed is relative and he maintains the obligatory tranquility, then prayer behind him is permissible, and the congregant must follow him while being careful to maintain the obligatory tranquility.