Musnad Ahmad 32:1791

Aswad ibn Amir and Affan narrated to us, they said: Aban narrated to us, from Qatadah, from Anas. Aswad said: Anas ibn Malik narrated to us, that the Prophet, peace and blessings of Allah be upon him, said: "Align your rows, bring them close together, and keep the necks in line. For by the One in Whose hand is the soul of Muhammad, I see the devils entering through the gaps in the row as if they were small black sheep." Affan said: "I see the devil entering."