The Prophet ﷺ said, "By Him in whose hand is the soul of Muhammad, no one of this community, Jew or Christian, hears of me and then dies without believing in what I was sent with, except that he will be among the people of the Fire."
وبإسناده عن النبي ﷺ أنه قال والذي نفس محمد بيده لا يسمع بي أحد من هذه الأمة يهودي أو نصراني ثم يموت ولا يؤمن بالذي أرسلت به إلا كان من أصحاب النار
Its isnad is sahih. "By the One in whose hand is the soul of Muhammad" — what is meant by "the soul" here is the spirit or the self, and the oath is meant for emphasis and to show greater care and concern with the matter.