Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that when the Prophet (peace and blessings be upon him) began the prayer, he would say: 'Subhanaka Allahumma wa bihamdika, wa tabaraka ismuka, wa ta'ala jadduka, wa la ilaha ghayruka' (O Allah, You are free of all imperfection, and with Your praise; Your name is blessed, Your majesty is exalted, and there is no god other than You).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10081