We said, "Messenger of Allah, this greeting of peace upon you we have already learned. But how do we send blessings upon you?"He replied, "Say, 'O Allah, send blessings upon Muhammad, Your servant and Messenger, as You sent blessings upon Ibrahim, and bestow favor upon Muhammad and the family of Muhammad as You bestowed favor upon Ibrahim and the family of Ibrahim.'"
حدثنا عبد الملك بن عمرو قال حدثنا عبد الله بن جعفر الزهري عن يزيد بن عبد الله بن الهاد عن عبد الله بن خباب
قلنا يا رسول الله هذا السلام عليك قد علمناه فكيف الصلاة عليك قال قولوا اللهم صل على محمد عبدك ورسولك كما صليت على إبراهيم وبارك على محمد وآل محمد كما باركت على إبراهيم وآل إبراهيم
Its isnad is sahih. Abdullah is Abu Amir Aqadi, mentioned many times before. Abdullah bin Jafar Zuhri, who is not the actual Zuhri, is thiqah, and Abdullah bin Khabab bin Aratt is thiqah, one of the senior Tabiin.Bukhari related the hadith, 8/532, no. 4797, Fath, in the tafsir of "Indeed, Allah and His angels send blessings upon the Prophet" [Ahzab 33:56], and Nasai, 3/49, no. 1293, and Ibn Majah, 1/292, no. 903.