It was narrated from Muhammad bin Maslamah that:

When the Messenger of Allah ﷺ stood to offer a voluntary prayer he would say: " Allahu Akbar Wajahtu wajhi lilladhi fataras-samawatiwal-arda hanifan musliman wa ma ana minal-mushrikin. Inna salati wa nusuki wa mahyaya wa mamati lillahi rabbil-alamin, la sharika lahu, wa bidhalika umirtu wa ana awwalul-muslimin. Allahumma antal-maliku la ilaha illa anta subhanaka wa bihamdik (Allah is Most Great. Verily, I have turned my face toward Him who created the Heavens and the Earth hanifa (worhsipping none but Allah Alone), as a Muslim, and I am not of the idolaters. Verily, my Salah, my sacrifice, my living, and my dying are for Allah, the Lord of the all that exists. He has no partner. And of this I have been commanded, and I am the first of the Muslims. O Allah, You are the Sovereign and there is none worthy of worship but You, glory and praise be to You.)" Then he would recite.

أَخْبَرَنَا يَحْيَى بْنُ عُثْمَانَ الْحِمْصِيُّ، قَالَ حَدَّثَنَا ابْنُ حِمْيَرٍ، قَالَ حَدَّثَنَا شُعَيْبُ بْنُ أَبِي حَمْزَةَ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، وَذَكَرَ، آخَرَ قَبْلَهُ عَنْ عَبْدِ الرَّحْمَنِ بْنِ هُرْمُزَ الأَعْرَجِ، عَنْ مُحَمَّدِ بْنِ مَسْلَمَةَ، أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ إِذَا قَامَ يُصَلِّي تَطَوُّعًا قَالَ " اللَّهُ أَكْبَرُ وَجَّهْتُ وَجْهِيَ لِلَّذِي فَطَرَ السَّمَوَاتِ وَالأَرْضَ حَنِيفًا مُسْلِمًا وَمَا أَنَا مِنَ الْمُشْرِكِينَ إِنَّ صَلاَتِي وَنُسُكِي وَمَحْيَاىَ وَمَمَاتِي لِلَّهِ رَبِّ الْعَالَمِينَ لاَ شَرِيكَ لَهُ وَبِذَلِكَ أُمِرْتُ وَأَنَا أَوَّلُ الْمُسْلِمِينَ اللَّهُمَّ أَنْتَ الْمَلِكُ لاَ إِلَهَ إِلاَّ أَنْتَ سُبْحَانَكَ وَبِحَمْدِكَ " . ثُمَّ يَقْرَأُ .