The ruling on one who limits the tashahhud to saying: “O Allah, send blessings and peace upon Muhammad”?

Question:
He says: Honorable Shaykh, may Allah grant you success, you mentioned (may Allah preserve you) in your book *Al-Mukhtasar al-Fiqhi* regarding the final tashahhud that it is obligatory to say “O Allah, send blessings upon Muhammad,” and that what remains is a recommended practice (sunnah). So if the one praying limits himself to that and does not recite the complete Ibrahimic prayer?

Answer:
The pillar (rukn) is to say “O Allah, send blessings and peace upon Muhammad”; this fulfills the pillar. However, completing what was reported from the Messenger (peace and blessings be upon him) is better, so one recites the complete Ibrahimic prayer. But if we suppose that a person did not do this, do we invalidate his prayer, even though he has sent blessings upon the Prophet (peace and blessings be upon him)? No, we say he has fulfilled the pillar. Yes.