When is a rak'ah considered to have been caught in congregation: Is it a condition for the worshipper to catch the imam in his bowing with a state of tranquility, or is it sufficient for the worshipper to enter the prayer while the imam is bowing before he rises from his bowing?

To catch the rak'ah through bowing (ruku'), the ma'mum (follower) must join the imam before the imam rises from the minimum sufficient amount of bowing, which is the bending where one can place the hands on the knees. So, if one catches the imam while he is in the state of tranquility (ṭuma'nīnah) during bowing, or reaches the sufficient amount of bowing before the imam rises from that sufficient amount, then the rak'ah is counted. However, if the ma'mum is bowing while the imam is rising, the rak'ah is not counted, and the ma'mum should follow the imam into prostration (sujud). Tranquility (ṭuma'nīnah) is the stillness and steadiness of the limbs while performing the physical pillars.