I read in al-Tabaqaat by al-Taaj al-Subki, quoting from his father, the words: “The best words with which a person can send blessings upon the Prophet (peace and blessings of Allaah be upon him) are the words that appear in the Tashahhud.
He said: Whoever says these words has sent blessings upon the Prophet (peace and blessings of Allaah be upon him) for certain, and whoever says any other words, there is some doubt as to whether he has sent blessings in the required manner, because they said, ‘How should we send blessings upon you?’ and he said, ‘Say…’ So he stated that sending blessings upon him is done in that manner.