Hadithcore
Mishkat al-Masabih, 821

Muhammad b. Maslama said that when God’s Messenger stood up to observe voluntary prayers he said, “God is most great. I have turned my face as a <i>hanif</i> towards Him who created the heavens and the earth, and I am not a polytheist.” And he mentioned the tradition like that of Jabir, except that he said, “l am of the Muslims.” Then he said, “O God, You are the King. There is no god but You. Glory be to You, and with praise of You [do I begin my worship].” Then he would recite [some verses]. Nasa’i transmitted it.

وَعَن مُحَمَّد بن مسلمة قَالَ: إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ إِذَا قَامَ يُصَلِّي تَطَوُّعًا قَالَ: «اللَّهُ أَكْبَرُ وَجَّهْتُ وَجْهِيَ لِلَّذِي فَطَرَ السَّمَاوَاتِ وَالْأَرْض حَنِيفا مُسلما وَمَا أَنَا مِنَ الْمُشْرِكِينَ» . وَذَكَرَ الْحَدِيثَ مِثْلَ حَدِيثِ جَابِرٍ إِلَّا أَنَّهُ قَالَ: «وَأَنَا مِنَ الْمُسْلِمِينَ» . ثُمَّ قَالَ: «اللَّهُمَّ أَنْتَ الْمَلِكُ لَا إِلَهَ إِلَّا أَنْتَ سُبْحَانَكَ وَبِحَمْدِكَ» ثُمَّ يَقْرَأُ. رَوَاهُ النَّسَائِيّ

References1 variant
In-Book Reference
Book 4, Hadith 248