Musannaf Ibn Abi Shayba 1:1731

Waki’ narrated to us, from Isma’il bin Abi Khalid, from al-Zubayr bin ‘Adi, from Mus’ab bin Sa’d, who said: I was holding the Mushaf for my father, and I inserted my hand like this—meaning he touched his penis—so he said to him: "Perform wudu."