The Messenger of Allah ﷺ said, "When one of you drinks, he should not breathe into the vessel. When one of you goes to the lavatory, he should not touch his private parts with his right hand, nor wipe himself with his right hand."
قال رسول الله ﷺ: إذا شرب أحدكم فلا يتنفس في الإناء، وإذا أتى أحدكم الخلاء فلا يمس ذكره بيمينه، ولا يتمسح بيمينه.
Bukhari narrated it in the Book of Wudu (153) and Muslim in the Book of Purification (267), both through Yahya bin Abu Kathir, from Abdullah bin Abu Qatada, in a similar wording. In one version reported by Muslim it reads, "and that he should cleanse himself with his right hand." His saying "and he should not wipe himself with his right hand" means he should not use it to clean himself after relieving himself.