Musannaf Ibn Abi Shayba 3:6196

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Hammam, from Qatadah, from Yazid ibn Abdullah ibn al-Shikhkhir, from Abdullah ibn Amr, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever reads the Quran in less than three [days] has not understood it."