Sahih Muslim , Book 2, Hadith 512 (USC-MSA) ⓘ
Abu Qatada reported it from his father that the Messenger of Allah said: When anyone amongst you enters the privy he should not touch his penis with his right hand.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا وَكِيعٌ، عَنْ هِشَامٍ الدَّسْتَوَائِيِّ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ عَبْدِ اللَّهِ بْنِ أَبِي قَتَادَةَ، عَنْ أَبِيهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا دَخَلَ أَحَدُكُمُ الْخَلاَءَ فَلاَ يَمَسَّ ذَكَرَهُ بِيَمِينِهِ " .
also referenced under: Book 2, Hadith 81 (In-Book) & Hadith 267 (Reference)