I heard the Messenger of Allah (ﷺ) say: Straighten your rows, or Allah would create dissension amongst you.
Sahih Muslim Book 4, Hadith 140
← Previous Hadith This is what was transmitted to us by Abu Huraira from the Messenger of Allah (ﷺ) and, while making a mention of a few ahadith, said: (The Messengerof Allah directed us thus): Establish rows in prayer, for the making of a row (straight) is one of the merits of prayer.

Sahih Muslim Book 4, Hadith 139 Next Hadith → The Messenger of Allah (may peace-be upon him) used to straighten our rows as it lie were straightening an arrow with their help until be saw that we had learnt it from him. One day he came out, stood up (for prayer) and was about to say: Allah is the Greatest, when he saw a man, whose chest was bulging out from the row, so he said: Servants of Allah, you hint straighten your rows or Allah would create dissension amongst you.

Sahih Muslim Book 4, Hadith 141