Musannaf Ibn Abi Shayba 5:406

Abu Bakr narrated to us, he said: Muhammad ibn Yazid narrated to us, from al-Nu’man, from Makhul, he said: A man asked him about Zakat, so he said: "Pay it to the Imam." And he said: "The Imam is the Qur'an," and he used to conceal that.