Musannaf Ibn Abi Shayba 1:37

Affan narrated to us, he said: Aban al-Attar narrated to us, from Yahya ibn Abi Kathir, from Zayd ibn Abi Salam, from Abu Malik al-Ash'ari, that the Messenger of Allah, may Allah bless him and grant him peace, used to say: "Purification is half of faith."