Musannaf Ibn Abi Shayba 1:641

Ibn Abi Shaybah narrated to us, he said: Ibn ‘Ulayyah informed us, from Ayyub, from Ibn Sirin, who said: I was informed that Ibn ‘Abbas drank milk, and they mentioned to him [the need for] ablution and rinsing the mouth. He said: "I do not care about it; [it is] a matter of ease, may it be made easy for you."