Riyad as-Salihin 1096
Abu Hurairah (May Allah be pleased with him)reported:
The Messenger of Allah (ﷺ) said, "Let the Imam stand in the middle (so that those praying behind him should be standing both on his right and his left) and close the gaps." [Abu Dawud] .
وعن أبي هريرة رضي الله عنه قال قال رسول الله صلى الله عليه وسلم‏:‏ ‏
"‏وسطوا الإمام، وسدوا الخلل‏"‏ ‏(‏‏(‏رواه أبو داود‏)‏‏)‏‏.‏