Musnad Ahmad 31:454

Waki’ narrated to us, from ‘Ikrimah ibn ‘Ammar, from ‘Asim ibn Shumaykh, from Abu Sa’id, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would exert himself in an oath, he would say: "No, by the One in Whose hand is the soul of Abu al-Qasim."