Sunan an Nasai
845
Sahih+2

It was narrated that Anas said:"The Messenger of Allah (ﷺ) turned to face us when he stood up to pray, before he said Takbir, and said: 'Make your rows straight and fill the gaps, for I can see you from behind my back

SahihAbu Ghuddah
SahihAl-Albani