Sunan an-Nasai Book 12, Hadith 1052

أَخْبَرَنَا يَحْيَى بْنُ عُثْمَانَ، قَالَ حَدَّثَنَا ابْنُ حِمْيَرٍ، قَالَ حَدَّثَنَا شُعَيْبٌ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، وَذَكَرَ، آخَرَ قَبْلَهُ عَنْ عَبْدِ الرَّحْمَنِ الأَعْرَجِ، عَنْ مُحَمَّدِ بْنِ مَسْلَمَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا قَامَ يُصَلِّي تَطَوُّعًا يَقُولُ إِذَا رَكَعَ ‏ "‏ اللَّهُمَّ لَكَ رَكَعْتُ وَبِكَ آمَنْتُ وَلَكَ أَسْلَمْتُ وَعَلَيْكَ تَوَكَّلْتُ أَنْتَ رَبِّي خَشَعَ سَمْعِي وَبَصَرِي وَلَحْمِي وَدَمِي وَمُخِّي وَعَصَبِي لِلَّهِ رَبِّ الْعَالَمِينَ ‏"‏ ‏.‏

It was narrated from Muhammad bin Maslamah that:When the Messenger of Allah (ﷺ) stood to offer a voluntary prayer, he would say when he bowed: "Allahumma laka rak'atu wa bika amantu wa laka aslamtu wa alayka tawwakaltu, anta Rabbi, khasha'a sam'i wa basri wa lahmi wa dammi wa mukhi wa 'asabi Lillahi Rabbil-'Alamin ( O Allah, to You I have bowed, in You I believe, to You I have submitted and in You I put my trust. You are my Lord. My hearing, my sight, my flesh, my blood, my brain and my sinews are humbled before Allah, the Lord of the Worlds)