Musnad Ahmad 459:8

Isma'il narrated to us, from Hisham al-Dastawa'i, from Yahya ibn Abi Kathir, from Muhammad ibn Ibrahim ibn al-Harith, from Khalid ibn Ma'dan, from al-'Irbad ibn Sariyah, that he narrated to them: "That the Messenger of Allah (peace and blessings of Allah be upon him) used to seek forgiveness for the first row three times, and for the second row once."