Musnad Ishaq

Benefits:
From the aforementioned hadith, it is understood that the straightening of the rows is part of the beauty of the prayer. However, the beauty intended here is not merely the outward beauty that results from the arrangement of the rows in prayer, such that the rich and the poor, the learned and the unlearned, the black and the white, the handsome and the unattractive—all standing in the same row—appear good together. Rather, the beauty intended here is the true (inner) beauty. (Irshad al-Sari: 2/66)

In another hadith, it is narrated that Sayyiduna Anas bin Malik radi Allahu anhu said: The Messenger of Allah sallallahu alayhi wa sallam said: “Straighten your rows, for indeed straightening the rows is part of perfecting the prayer.” (Bukhari, Kitab al-Adhan, no. 723)

Sayyiduna Nu’man bin Bashir radi Allahu anhu said: The Messenger of Allah sallallahu alayhi wa sallam would straighten our rows as if he were straightening an arrow with it, until he was satisfied that we had understood this matter well from him. One day, he came to the prayer mat and saw a man whose chest was protruding forward. The Messenger of Allah sallallahu alayhi wa sallam said: “O servants of Allah! Straighten your rows, or Allah will cause discord among you.” (Muslim, Kitab al-Salat, no. 979)

Method of joining the rows: ... It is narrated from Sayyiduna Anas radi Allahu anhu that the Noble Prophet sallallahu alayhi wa sallam said: “Straighten your rows, for I can see you from behind my back.” (Upon hearing this command of the Noble Prophet sallallahu alayhi wa sallam) each one of us would place his shoulder against the shoulder of his companion and his foot against his foot in the row. (Bukhari, Kitab al-Adhan, no. 725)