"The Prophet prohibited that a man should touch his penis with his right hand."
Jami` at-Tirmidhi Book 1, Hadith 15
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Jami` at-Tirmidhi Book 1, Hadith 14 Next Hadith → "They said to Salman, 'Your Prophet taught you about everything, even defecating?' So Salman said, 'Yes. He prohibited us from facing the Qiblah when defecating and urinating, performing Istinja with the right hand, using less than three stones for Istinja, and using dung or bones for Istinja"

Jami` at-Tirmidhi Book 1, Hadith 16