Is saying "Allahumma salliala Muhammada waAla Aali Muhammad" enough to fulfill the Sunnah of sending blessings on the prophet, and is it allowed to say this in prayer after Al-Attahiyat?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The most complete manner of sending salutations upon the Prophet is the well-known Abrahamic salutation that he taught to his companions, but the sufficient way of sending salutations as explained by the jurists in prayer and outside the prayer is saying: “Allahumma salli ‘ala Muhammad.” (O Allah! Bestow your Blessings and Peace upon [Prophet] Muhammad.).

Therefore, whoever says (this salutation): “allahumma salli ‘ala Muhammad.” only, has asked Allah to bestow His Blessings and Peace upon him, and the obligation has been fulfilled if he is in prayer, and the Sunnah has been fulfilled if he is not praying.

For more benefit on the shortest form of Tashahhud and its different wordings, please refer to Fatawa 371557, 322516 and 375343.

Allah knows best.