Riyad us-Salihin Hadith 160

Messenger of Allah (ﷺ) said, "Straighten your rows (during Salat) or Allah would create dissension amongst you". [Al-Bukhari and Muslim]. In another narration reported by Muslim, Nu'man bin Bashir (May Allah be pleased with them) narrated: Messenger of Allah (ﷺ) used to straighten our rows (in Salat), as if he was straightening an arrow with their help until he saw that we had learnt it from him. One day he came out, stood up (for Salat) and was about to say: Allahu Akbar (Allah is the Greatest), (marking the beginning of the prayer) when he saw a man, whose chest was bulging out from the row. He said, "Slaves of Allah, you must straighten your rows or Allah would create dissension amongst you."