Sayyiduna Abu Sa‘id Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When you stand (for prayer), straighten your rows and fill the gaps, for indeed I see you from behind my back."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب قيام المأمومين خلف الإمام وما فيه من السنن / 1548