It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Close up your rows, keep them near one another, and align your necks. By Allah, I see Satan entering through the gaps in the rows like a young lamb."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5179