Sayyiduna Abu Umamah (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "Indeed, Allah Almighty and His angels send mercy upon those in the first row." The people said: "O Messenger of Allah, and upon the second row too?" The Prophet (peace and blessings be upon him) said: "Indeed, Allah Almighty and His angels send mercy upon those in the first row." The people said: "O Messenger of Allah, and upon the second row too?" The Prophet (peace and blessings be upon him) said: "Upon the second too." And the Prophet (peace and blessings be upon him) said: "Keep your rows straight, keep your shoulders level, be gentle in the hands of your brothers, and close the gaps, for Satan enters in between you as though he were a small goat." The narrator says: What is meant is as though he were the young of a small goat.
Ahmad and al-Tabarani (in al-Mu'jam al-Kabir) recorded it, and Ahmad's narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2509