It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Indeed, Allah and His angels send blessings upon the first row. Straighten your rows, keep your shoulders level, be gentle with the hands of your brothers, and fill the gaps, for Satan enters between you like hadhaf."
(The narrator said:) Hadhaf means small lambs.
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7696