Musnad Ahmad

It is narrated from Sayyiduna Ali (may Allah be pleased with him) that when the Prophet (peace and blessings of Allah be upon him) would go into bowing (ruku‘), he would recite this supplication: «اَللّٰهُمَّ لَكَ رَكَعْتُ وَبِكَ آمَنْتُ وَلَكَ أَسْلَمْتُ أَنْتَ رَبِّي خَشَعَ سَمْعِي وَبَصَرِي وَمُخِّي وَعَظْمِي وَعَصَبِي وَمَا اسْتَقَلَّتْ بِهِ قَدَمِي لِلّٰهِ رَبِّ الْعَالَمِينَ.» "O Allah! I have bowed for You, believed in You, submitted to You; You alone are my Lord. My ears, my eyes, my mind, my bones, and my tendons are humbled before You, and my feet are also humbled for the sake of Allah, Lord of the worlds."