Musannaf Ibn Abi Shayba 1:642

Waki’ narrated to us, from Qurrah ibn Khalid, from Yazid, from his brother Mutarrif ibn al-Shikhkhir, who said: I drank pure milk after I had performed ablution, so I asked Ibn Abbas. He said: "I do not care about it at all; be lenient, and it will be lenient for you."