Yahya related to me from Malik from Ismail ibn Muhammad ibn Sadibn Abi Waqqas that Musab ibn Sad ibn Abi Waqqas said, "I was holdingthe Book for Sad ibn Abi Waqqas and I rubbed myself. Sad said, 'Didyou touch your penis?' I replied, 'Yes,' and he said, 'Get up and dowudu.' So I got up and did wudu and then returned."
USC-MSA web (English) reference: Book 2, Hadith 61
Arabic reference: Book 2, Hadith 91