Musannaf Ibn Abi Shayba 1:737

Muhammad ibn Fudayl narrated to us, from al-Ala’ ibn al-Musayyab, from Fudayl ibn Amr, who said: Umar said: "When you perform ghusl (full-body ablution) for janabah (major ritual impurity), then rinse your mouth three times, for it is more thorough."