Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "When your Prophet (peace and blessings be upon him) was bowing or prostrating, he would say: 'Subhanaka wa bihamdika, astaghfiruka wa atubu ilayk' (Glory be to You and with Your praise; I seek Your forgiveness and I turn to You in repentance)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10264