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."' (JA)
English reference (JA): Vol. 3, Book 11, Hadith 2091 (JA)
Arabic reference (JA): Book 11, Hadith 2169 (JA)