The ruling on praying alone behind the row

Question:
This questioner says in his second question: A man prayed one rak'ah alone behind the row, and when the imam finished the prayer, he came to the fifth [rak'ah].

Answer:
It appears that the prayer is not valid, because the Messenger (peace and blessings be upon him) said, "There is no prayer for one who is isolated in the row." So if he prays one rak'ah or more, he has no prayer. However, if he prays part of it, such as standing behind the row and then joining and reciting, or entering the row, or bowing and then entering the row, or joining with another, there is no harm. But if he completes the rak'ah, it is not valid, due to the saying of the Messenger (peace and blessings be upon him) and he approved Abu Bakr when he bowed, joined the row, and then entered the row, and he did not order him to make it up. This indicates that if the isolation is in the bowing or in the standing before the bowing, this suffices, then he enters the row or another joins him in the row. But if he prostrates and no one is with him, or he does more than that, then the prayer is not valid. And the Prophet (peace and blessings be upon him) saw and said, "There is no prayer for one who is isolated behind the row." May Allah send prayers and blessings upon him. May Allah send prayers and blessings, and increase and bless our Prophet and our beloved Muhammad.