Riyad as-Salihin: The Book of Virtues – Hadith No. 413 — Anas (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said… (Hadith No. 413)

Anas (May Allah be pleased with him) reported: 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).".

Explanation:

Reference: Riyad as-Salihin, The Book of Virtues, Hadith No. 413