It was narrated that Rifa'ah bin 'Arabah A-Juhani said:
"The swearing of the Messenger of Allah (ﷺ) when he took an oath and I bear witness before Allah was: 'By the One in Whose Hand is my soul."' (ID)
English reference (ID): Vol. 3, Book 11, Hadith 2091 (ID)
Arabic reference (ID): Book 11, Hadith 2169 (ID)