ʿAbd al-ʿAzīz ibn Suhayb, from Anas ibn Mālik—may Allah be pleased with him—who said: The Messenger of Allah, may Allah bless him and grant him peace, said:

“Complete the rows, for I can see you behind my back.”

In another wording:

“Straighten the rows.”

Al-Bukhārī narrated through Humayd, from Anas, who said:

“The call to commence prayer was given, and the Messenger of Allah, may Allah bless him and grant him peace, turned towards us and said: ‘Straighten your rows and stand close together, for I can see you behind my back.’ Each of us would place his shoulder against his companion’s shoulder and his body against his body.”

If straightening and aligning the row is part of perfecting and establishing prayer—such that, if they abandoned alignment and evenness altogether, with one person’s head level with the lower half of another’s body, they would not constitute a row and would be ordered to repeat their prayer, with even greater reason than the man who prayed alone behind the row and whom the Prophet, may Allah bless him and grant him peace, ordered to repeat his prayer—then how much more important is it to perform the actions of prayer correctly and in a balanced manner, rather than failing to keep one’s back properly aligned in bowing and prostration?

This is also supported by the following report, which constitutes independent evidence on this issue. Both compilers narrated in their Sahīh collections, through Shuʿba, from Qatāda, from Anas—may Allah be pleased with him—that the Prophet, may Allah bless him and grant him peace, said:

“Perform bowing and prostration properly, for, by Allah, I can indeed see you behind me”—in another narration, “behind my back”—“when you bow and prostrate.”

In another narration recorded by al-Bukhārī, through Hammām, from Qatāda, from Anas—may Allah be pleased with him—he heard the Prophet, may Allah bless him and grant him peace, say:

“Complete your bowing