Mishkat al-Masabih 1097
An-Nu‘man b. Bashir said:
God’s Messenger used to straighten our rows when we stood up to pray, and when we were straight he said, “God is most great." 

Abu Dawud transmitted it.
وَعَنِ النُّعْمَانِ بْنِ بَشِيرٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُسَوِّي صُفُوفَنَا إِذَا قُمْنَا إِلَى الصَّلَاةِ فَإِذَا اسَتْوَيْنَا كَبَّرَ. رَوَاهُ أَبُو دَاوُد