What is the ruling on saying "Alhamdulillah" (all praise is due to Allah) behind the Imam before he finishes saying "Sami'a Allahu liman hamidah" (Allah hears those who praise Him)? And is it obligatory to wait for the Imam to finish "Sami'a Allahu liman hamidah" before saying "Alhamdulillah"? What is the ruling on a worshipper forgetting to say "Alhamdulillah" until he has straightened up, and what is the ruling on one who starts saying "Alhamdulillah" during the rising and finishes it after straightening up?

The congregant says "Rabbana wa lakal-hamd" (Our Lord, and to You be all praise) after the Imam says "Sami‘ Allahu liman hamidah" (Allah hears the one who praises Him). It is preferable for this to be said while rising from bowing, and it is permissible to complete it after standing upright. It is not obligatory to wait for the Imam to finish his saying. If the congregant forgets to say the tahmid until he has stood upright from bowing, then its place has passed, but his prayer is valid, and he is not required to perform sujud as-sahw (prostration of forgetfulness).