It was narrated that Rifa'ah Al - Juhani said:
"When the Prophet (ﷺ) took an oath he would say: 'By the One in Whose Hand is the soul of Muhammad."' (IT)
English reference (IT): Vol. 3, Book 11, Hadith 2090 (IT)
Arabic reference (IT): Book 11, Hadith 2168 (IT)