What is the ruling on the prayer of one who did not recite the Fatihah completely during prayer, due to being unable to catch up with the Imam?

The recitation of Al-Fatihah in prayer is not valid except while standing, unless one is unable to do so. If the ma'mum (follower) does not find an opportunity to recite Al-Fatihah, he should follow his imam (leader) and bow with him, without delay. This is due to the obligation of following the imam. The ma'mum prioritizes following the imam and bowing with him, and should not recite Al-Fatihah before standing or be preoccupied with it to the extent that he misses the rak'ah. This is based on the Prophet's (peace and blessings be upon him) saying: "When one of you comes to prayer and the imam is in a certain state, let him do as the imam does."