Yahya related to me from Malik from Abdullah ibn Abi Bakr ibn Hazm that in a letter that the Messenger of Allah, may Allah bless him and grant him peace, sent to Amr ibn Hazm it said that no-one should touch the Qur'an unless he was pure.
USC-MSA web (English) reference: Book 15, Hadith 1
Arabic reference: Book 15, Hadith 473