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 while we were in the gathering of Sayyiduna Sa'd bin Ubadah (may Allah be pleased with him), and Bashir bin Sa'd said: O Messenger of Allah, Allah has commanded us to send blessings upon you, so how should we send blessings upon you? The Messenger of Allah (peace and blessings be upon him) remained silent until we wished he had not asked 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 and the family of Muhammad as You sent blessings upon Ibrahim among all the worlds; indeed You are Praiseworthy, Glorious. And as for the salam, you already know it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14797