Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) said: When this verse was revealed: "Indeed, Allah and His angels send blessings upon the Prophet" [Surah al-Ahzab: 56] (the rest of the hadith is the same).
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16336