Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) used to say: "Of the angels of Allah, the guard of the night and the guard of the day meet each other at the break of dawn; and if you wish, recite this verse: 'And (observe) the recitation of Fajr; indeed, the recitation of Fajr is witnessed (by the angels)' [Surah al-Isra: 78]."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9107