Musannaf Ibn Abi Shayba 7:333

Mu'awiyah ibn 'Amr narrated to us, from Za'idah, from Ibrahim ibn Muhajir, from 'Ikrimah ibn 'Ammar, from 'Asim ibn Shaykh, from Abu Sa'id al-Khudri, who said: When the Prophet, peace and blessings of Allah be upon him, would exert himself in an oath, he would say: "By the One in Whose hand is the soul of Abu al-Qasim."