Musnad Ahmad 28:70

Yazid narrated to us, Hammam informed us, from Qatadah, from Yazid ibn Abdullah ibn al-Shikhkhir, from Abdullah ibn Amr, who said: I said: "O Messenger of Allah, in how much time should I recite the Quran?" He said: "Recite it in every month." I said: "I am capable of more than that." He said: "Recite it in twenty-five [days]." I said: "I am capable of more than that." He said: "Recite it in twenty [days]." I said: "I am capable of more than that." He said: "Recite it in fifteen [days]." I said: "I am capable of more than that." He said: "Recite it in ten [days]." I said: "I am capable of more than that." He said: "Recite it in seven [days]." I said: "I am capable of more than that." He said: "He does not understand it, the one who recites it in less than three [days]."