Musannaf Ibn Abi Shayba 1:738

Abu Amir al-Aqdi narrated to us, from al-Zubayr, from Abdullah ibn Zuhayma, who said: My grandmother narrated to me that Uthman, when he would perform ghusl (full-body ablution) from janaba (major ritual impurity), would rinse his mouth and sniff water into his nose three times.