Yahya related to me from Malik from Ismail ibn Muhammad ibn Sad
ibn Abi Waqqas that Musab ibn Sad ibn Abi Waqqas said, "I was holding
the Book for Sad ibn Abi Waqqas and I rubbed myself. Sad said, 'Did
you touch your penis?' I replied, 'Yes,' and he said, 'Get up and do
wudu.' So I got up and did wudu and then returned."
Muwatta Malik Book 2, Hadith 60