Where does the first half of the Tashahhud end: is it with the words "And I bear witness that Muhammad is His slave and His Messenger" or with the words "O Allah, bless Muhammad and the family of Muhammad"?

The Tashahhud ends when the worshipper says: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and messenger." It has established forms, among which is what Ibn Mas'ud narrated: "All compliments, prayers and good things are due to Allah. Peace be upon you, O Prophet, and Allah's mercy and blessings. Peace be upon us and upon the righteous servants of Allah. I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and messenger." As for sending blessings upon the Prophet, it is not part of the Tashahhud. It is a pillar in the final Tashahhud, and there is a difference of opinion regarding its permissibility in the first Tashahhud.