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