Musnad Abu Ya'la al-Mawsili 140:44

Hafs ibn 'Abd Allah ibn 'Umar al-Hulwani narrated to us, Durust ibn Ziyad narrated to us, from Yazid al-Raqashi, from Anas, who said: I was walking with the Prophet, may Allah bless him and grant him peace, and he said to me: "O my son, ask for me from this house for water for ablution." I said: "The Messenger of Allah, may Allah bless him and grant him peace, is requesting water for ablution." They said: "Inform him that our bucket is [made of] the hide of a dead animal." He said: "Ask them, 'Have they tanned it?'" They said: "Yes." He said: "Then its tanning is its purification."