مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 340
Purification
How to act when relieving oneself - Section 1
وَعَنْ أَبِي قَتَادَةَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسلم: «إِذا شرب أحدكُم فَلَا ينتنفس فِي الْإِنَاءِ وَإِذَا أَتَى الْخَلَاءَ فَلَا يَمَسَّ ذَكَرَهُ بِيَمِينِهِ وَلَا يَتَمَسَّحْ بِيَمِينِهِ»
Abu Qatada reported God’s messenger as saying, "When one of you drinks he must not breathe into the vessel, and when he goes to relieve himself he must not touch his penis with his right hand, or wipe himself with his right hand.” (Bukhari and Muslim.)
Mishkat al-Masabih, 340مُتَّفَقٌ عَلَيْهِ (الألباني)