Musnad Abu Ya'la al-Mawsili 3:25

Abu Khaythama narrated to us, he said: Abd al-Rahman ibn Mahdi narrated to us, he said: Abd al-Aziz ibn Abd Allah ibn Abi Salamah narrated to us, he said: My paternal uncle, al-Majishun ibn Abi Salamah, narrated to me, from Abd al-Rahman al-A'raj, from Ubayd Allah ibn Abi Rafi', from Ali: That the Messenger of Allah (may Allah bless him and grant him peace), when he began the prayer, would say the takbir, then say: "I have turned my face toward He who originated the heavens and the earth, inclining toward truth, and I am not of the polytheists. Indeed, my prayer, my rites of sacrifice, my living, and my dying are for Allah, Lord of the worlds. There is no partner for Him, and with that I have been commanded, and I am the first of the Muslims." He said: And he mentioned the rest of the hadith.