Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"I asked Aishah: 'With what did the Prophet (saw) start voluntary prayers?' She said: 'You have asked me about something which no one has asked before. He used to say Allahu Akbar ten times, and Al-Hamdu Lillah ten times and Subhan Allah ten times, and he would say Allahumma aghfirli wahdini, warzuqni, wa 'afini (O Allah, forgive me, guide me, grant me provision and give me good health)," and he would seek refuge from the difficulty of the standing on the Day of Resurrection." Hasan