It is narrated from Abu Hurairah (may Allah be pleased with him) that he asked: O Messenger of Allah! How should we send blessings upon you in prayer? The Prophet ﷺ replied: "Say: O Allah! Send mercy upon Muhammad ﷺ and upon the family of Muhammad, as You sent mercy upon Ibrahim (peace be upon him), and send blessings upon Muhammad ﷺ and upon the family of Muhammad, as You sent blessings upon Ibrahim (peace be upon him). Then send peace upon me."
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 257
Hadith Takhrijاسنادہ ضعیف لشدۃ ضعف شیخ الشافعی، أخرجہ البیہقی فی معرفۃ السنن والآثار: 905۔