Narrated by Abdullah ibn Umar(Radhiallaho anho):

Allah’s Messenger (sallAllaahu `alaihi wasallam) said : Set the rows in order, stand shoulder to shoulder, close the gaps, be pliant in the hands of your brethren, and do not leave openings for the devil. If anyone joins up a row, Allah will join him up, but if anyone  breaks a row, Allah will cut him off.

Sunan of Abu-Dawood 666