When the Messenger of Allah ﷺ stood to pray a voluntary prayer, he would say, "Allah is the greatest. I have turned my face toward the One who created the heavens and the earth, devoted to the true faith as a Muslim, and I am not among those who associate partners with Allah. Indeed, my prayer, my rites of sacrifice, my living, and my dying are for Allah, Lord of all the worlds, who has no partner. With this I have been commanded, and I am the first of the Muslims. O Allah, You are the King. There is no god but You. Glory be to You, and praise be to You." Then he would begin reciting.
أخبرنا يحيى بن عثمان الحمصي قال حدثنا ابن حمير قال حدثنا شعيب بن أبي حمزة عن محمد بن المنكدر وذكر آخر قبله عن عبد الرحمن بن هرمز الأعرج
أن رسول الله ﷺ كان إذا قام يصلي تطوعا قال الله أكبر وجهت وجهي للذي فطر السموات والأرض حنيفا مسلما وما أنا من المشركين إن صلاتي ونسكي ومحياي ومماتي لله رب العالمين لا شريك له وبذلك أمرت وأنا أول المسلمين اللهم أنت الملك لا إله إلا أنت سبحانك وبحمدك ثم يقرأ
Sourcing, sahih. Tabarani transmitted it in Al-Kabir 19/231, 232, no. 515, from the hadith of Muhammad bin Humayr, with this wording, and its isnad is hasan. Part of it will come later, no. 1053. It has corroborating narrations, among them the previous hadith.