The Prophet ﷺ was asked, "We have been commanded to send blessings and peace upon you. As for the greeting of peace, we already know it, but how do we send blessings upon you?"He replied, "Say, 'O Allah, send blessings upon Muhammad as You sent blessings upon the family of Ibrahim. O Allah, send Your favor upon Muhammad as You sent Your favor upon the family of Ibrahim.'"
أخبرنا زياد بن يحيى قال حدثنا عبد الوهاب بن عبد المجيد قال حدثنا هشام بن حسان عن محمد عن عبد الرحمن بن بشر
قيل للنبي ﷺ أمرنا أن نصلي عليك ونسلم أما السلام فقد عرفناه فكيف نصلي عليك قال قولوا اللهم صل على محمد كما صليت على آل إبراهيم اللهم بارك على محمد كما باركت على آل إبراهيم
[Sahih]. It is in Al-Kubra, Hadith 1209. Hisham bin Hassan is a mudallis, as Ibn Madini, Abu Hatim, and others have said, but his hadith has many corroborating witnesses.