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 hanif 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, Thou art the King. There is no god but Thee. Glory be to Thee, and with praise of Thee .” Then he would recite . Nasa’i transmitted it.
Source: sunnah.com
Other translations
Arabic
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
sunnah.com