Musannaf Ibn Abi Shayba 7:342

Muhammad ibn Mus'ab narrated to us, from al-Awza'i, from Yahya ibn Abi Kathir, from Hilal ibn Abi Maymuna, from 'Ata' ibn Yasar, from Rifa'a al-Juhani, who said: When the Messenger of Allah (may Allah bless him and grant him peace) would swear an oath, he would say: "By the One in Whose hand is the soul of Muhammad."