Sahih Muslim 267b
Sahih Muslim · Book 2: The Book of Purification · The prohibition of cleaning oneself with the right hand
Abu Qatada reported it from his father that the Messenger of Allah (ﷺ) said:
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 " .
When anyone amongst you enters the privy he should not touch his penis with his right hand.