Musnad Abu Ya'la al-Mawsili 153:58

Mujahid ibn Musa narrated to us, Abu Nu'aym narrated to us, Sufyan narrated to us, from Musa, from Salim, from Ibn Umar, who said: The oath of the Prophet, may Allah bless him and grant him peace, by which he used to swear was: "No, and by the Turner of hearts."