Imam Al-Nawawi’s Riyad-us-Saliheen
Prohibition of using the right hand for cleaning after toilet without a valid
1648. Abu Qatadah (May Allah be pleased with him) said: The Prophet (sallallaahu alayhi wa sallam) said, “Do not touch your private parts with your right hand while urinating, nor for washing or cleaning (your private parts); and do not breathe into the drinking vessel from which you drink.”
[Al-Bukhari and Muslim].