Abu Salamah bin Abd al-Rahman (may Allah have mercy on him) said that Abu Hurayrah (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "The angels of the day and the night gather in the Fajr prayer." Then Abu Hurayrah (may Allah be pleased with him) said: Recite this verse if you wish: "And (establish) the recitation of dawn; indeed, the recitation of dawn is ever witnessed." [Surah al-Isra: 78]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2177