The World Federation One Stop Fiqh

Ruling 1403

If during prayers a follower makes the intention to perform the prayer on his own without any legitimate excuse, the validity of his congregational prayer is problematic [i.e. based on obligatory precaution, his congregational prayer is not valid]. However, his prayer [performed on his own, as opposed to his congregational prayer] is valid unless he does not act in accordance with the duty of one who performs prayers on his own, in which case, based on obligatory precaution, he must perform the prayer again. However, if he has added or omitted something, which if he had a legitimate excuse for doing would not invalidate the prayer, then it is not necessary for him to perform the prayer again. For example, if from the outset of the prayer he did not intend to perform it on his own, and he did not recite qirāʾah, and during rukūʿ he decided to perform the prayer on his own, then he can complete the prayer with the intention of performing the prayer on his own and it is not necessary for him to perform it again. The same applies if he performed an additional sajdah with the intention of following the congregation.