The Messenger of Allah ﷺ said, "Straighten your rows, draw them close together, and align your necks. By the One in whose hand my soul rests, I see Satan entering through the gaps in the row like little black sheep."
عن رسول الله ﷺ قال: رضوا صفوفكم، وقاربوا بينها، وحاذوا بالأعناق، فوالذي نفسي بيده إني لأرى الشيطان يدخل من خلل الصف كأنها الحذف.
Abu Dawud (667) and Nasai (815) both narrated it through Aban, from Qatada, from Anas bin Malik, who mentioned something similar. Its isnad is sahih. Aban is Yazid Attar Basri, a trustworthy narrator among those relied upon by Muslim, and Bukhari related from him in a suspended narration.Ibn Khuzayma (1545) and Ibn Hibban (2166) graded it sahih and narrated it in their two Sahihs, and Imam Ahmad (13735) narrated it as well, all of them through various routes from Aban with the same wording.As for al-hadhaf, these are small black sheep, the singular being hadhafa. In one version it is worded, "as if they were the young of hadhaf."