The Messenger of Allah ﷺ said, "Pack your rows tightly, draw them close together, and align your necks. By the One in whose hand my soul rests, I can see Satan slipping through the gaps in the row like little black sheep."
حدثنا مسلم بن إبراهيم حدثنا أبان عن قتادة
عن رسول الله ﷺ قال رصوا صفوفكم وقاربوا بينها وحاذوا بالأعناق فوالذي نفسي بيده إني لأرى الشيطان يدخل من خلل الصف كأنها الحذف
Its isnad is sahih. Nasai narrated it in the Book of Leadership in Prayer, the chapter on the imam urging the straightening of rows and bringing them close together, no. 816, from the hadith of Abu Aban bin Yazid Attar, with this wording. Ibn Khuzayma graded it sahih, no. 1545, as did Ibn Hibban, nos. 387 and 391. Qatada explicitly stated that he heard it directly, as recorded by Nasai. See also the following hadith.