Riyad us Saliheen: Hadith 1091

The Messenger of Allah (ﷺ) said, "Stand close together in your rows, keep nearer to one another, and put your necks in line, for by Him in Whose Hands my soul is, I see the Satan entering through the opening in the row like Al- hadhaf (i.e., a type of small black sheep found in Yemen)."

[Abu Dawud].