Sayyiduna Abu Mas'ud al-Ansari (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) came to us and sat with us in the gathering of Sayyiduna Sa'd bin Ubadah (may Allah be pleased with him). Sayyiduna Bashir bin Sa'd (may Allah be pleased with him), who is the father of Nu'man bin Bashir, said: Allah has commanded us to send blessings upon you, so how should we send blessings upon you? The narrator said: The Messenger of Allah (peace and blessings be upon him) remained silent until we wished he had not asked him this question. Then he (peace and blessings be upon him) said: "Say: O Allah, send Your mercy upon Muhammad and the family of Muhammad as You sent it upon Ibrahim, and send blessings upon Muhammad as You sent blessings upon Ibrahim among all the worlds; indeed You are Praiseworthy. And as for the salam, you already know it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14769