I asked the Messenger of Allah ﷺ, "How should we send blessings upon you?"He replied, "Pray and strive earnestly, then say, 'O Allah, bless Muhammad and the family of Muhammad, as You blessed the family of Ibrahim. Indeed, You are Praiseworthy and Glorious.'"
سألت رسول الله ﷺ كيف الصلاة عليك؟ قال: صلوا واجتهدوا، ثم قولوا: اللهم بارك على محمد وعلى آل محمد، كما باركت على آل إبراهيم إنك حميد مجيد.
Ahmad (1714) narrated it from Ali bin Bahr, who said, Isa bin Yunus narrated to us, Uthman bin Hakeem narrated to us, Khalid bin Salama narrated to us, that Abdul-Hamid bin Abdul-Rahman invited Musa bin Talha when he held a wedding celebration for his son. He said, "O Abu Isa, how did the account reach you regarding sending blessings on the Prophet ﷺ?"Musa said, "I asked Zayd bin Kharija about sending blessings on the Prophet ﷺ, and Zayd said, 'I asked the Messenger of Allah ﷺ himself, how should we send blessings on you?'" Then he related the hadith.Nasai also narrated it (1292) through the route of Uthman bin Hakeem, except that he abridged it. Its chain of transmission is sahih.