It is held by al-Sarakhsi in al-Mabsut...

It is held by al-Sarakhsi in al-Mabsut, Ibn Hammam in Sharh Fath al-Qadir, al-Qastallani in Irshad al-sari, and al-Qadi ‘Iyad in al-Shifa’. The only exception from this, being al-Shafi’i who reported that the Prophet said: Whoever sends not blessing on me, his salat is verily invalid. But he has neither authority for proving this saying, nor a sunnah to be followed.

And he was reviled by a group of ulama’ among whom being al-Tabari al-Qushayri, with being disapproved by a follower of his same school of thought, called al-Khitabi who said: It is not obligatory, and I have no knowledge of any other example (qudwah) confirming…