It was narrated from 'Abdullah bin Abi Qatadah that his father said:
"The Messenger of Allah (ﷺ) said: 'When any one of you enters Al-Khala' (the toilet), let him not touch his penis with his right hand.'" (RU)
Arabic/English book reference (RU): Vol. 1, Book 1, Hadith 25 (RU)