One who prays behind the rows, even for a single rak'ah, has an invalid prayer

Q: A man entered the prayer alone, and in the second rak'ah, another person joined him. After the imam's taslim, he stood up and performed a fifth rak'ah, considering the first rak'ah invalid because he performed it alone behind the rows. Is his prayer valid? And how should one act if such a situation occurs?

A: It is established from the Prophet (peace and blessings be upon him) that he said: It is also established from him (peace and blessings be upon him) that he saw a man praying alone behind the rows and ordered him to repeat the prayer. However, if one bows down behind the rows and then enters the rows before prostrating, his rak'ah is sufficient. This is based on what al-Bukhari narrated in his Sahih that Abu Bakrah al-Thaqafi (may Allah be pleased with him) came to the mosque while the Prophet (peace and blessings be upon him) was bowing. He bowed down behind the rows and then entered the rows, and the Prophet (peace and blessings be upon him) said to him: He did not order him to make up the rak'ah, which indicates that it is sufficient and that such an action is an exception to his (peace and blessings be upon him) statement: . And success is from Allah.