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], we said: "The salutation we have come to know; how shall we send blessings upon you?" Then the narrator mentioned a similar hadith.
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16328