When the Messenger of Allah ﷺ swore an oath, his oath was to say, "No, and I seek Allah's forgiveness."
حدثنا محمد بن عبد العزيز بن أبي رزمة أخبرني زيد بن حباب أخبرني محمد بن هلال حدثني أبي أنه سمع
كانت يمين رسول الله ﷺ إذا حلف يقول لا وأستغفر الله
No references are available.
[Its chain is daif] Ibn Majah reported it in the Book of Expiations, Chapter on the Oath the Messenger of Allah ﷺ Used to Swear By, hadith no. 2093, from the narration of Muhammad bin Hilal, from him. Hilal bin Abu Hilal Madani, the freedman of Banu Kaab, is obscure and no one other than Ibn Hibban has affirmed his reliability.