How to send blessing on him
They asked, because they already knew how to greet him.
بَارِكْ عَلَى مُحَمَّدٍ وَأَزْوَاجِهِ وَذُرِّيَّتِهِ، كَمَا بَارَكْتَ عَلَى آلِ إِبْرَاهِيمَ، إِنَّكَ حَمِيدٌ مَجِيدٌ
Bārik ʿalā Muḥammadin wa azwājihi wa dhurriyyatih, kamā bārakta ʿalā āli Ibrāhīm, innaka ḥamīdun majīd
“Send blessing on Muhammad and his wives and his offspring, as You blessed the family of Ibrahim. You are Praiseworthy, Glorious.”
Where this comes from
They asked him how to send blessing on him, since they had already been taught how to send peace. The answer he gave does not ask for anything new. It asks for the same thing that was already given to somebody else.
The full narration
Narrated Abu Humaid As-Sa`idi:The people asked, "O Allah's Messenger (ﷺ)! How shall we (ask Allah to) send blessings on you?" Allah's Apostle replied, "Say: O Allah! Send Your Mercy on Muhammad and on his wives and on his offspring, as You sent Your Mercy on Abraham's family; and send Your Blessings on Muhammad and on his offspring, as You sent Your Blessings on Abraham's family, for You are the Most Praiseworthy, the Most Glorious
Source
Ṣaḥīḥ al-Bukhārī · 3369
- Agreed upon as authentic (al-Bukhārī / Muslim)