Sunan Abu Dawud
665
Sahih+2

Narrated An-Nu'man ibn Bashir: The Messenger of Allah (ﷺ) used to straighten our rows when we stood up to pray, and when we were straight, he said: Allah is most great (takbir)

SahihAl-Albani
SahihMuhammad Muhyi Al-Din Abdul Hamid