It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Keep these bodies pure, may Allah purify you, for no servant spends the night in a state of purity except that an angel spends the night with him in his garment; at no hour of the night does he turn over except that the angel says: 'O Allah, forgive Your servant, for he spent the night in a state of purity.'"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13563