Musnad Ahmad 962:43

Ishaq ibn Yusuf narrated to us, Sharik narrated to us, from Ya'la ibn 'Ata, from a man who narrated to him that he heard Abu Umamah al-Bahili saying: When the Prophet of Allah, peace and blessings of Allah be upon him, would stand for prayer, he would say "Allahu Akbar" (Allah is the Greatest) three times. Then he would say, "La ilaha illa Allah" (There is no god but Allah) three times. Then he would say, "Subhan Allah wa bihamdihi" (Glory be to Allah and by His praise) three times. Then he would say, "I seek refuge in Allah from the accursed Satan, from his madness (hamz), his blowing (nafkh), and his spitting (nafth)."