Musnad Ahmad 28:30

Isma'il narrated to us, 'Ata' ibn al-Sa'ib informed us, from his father, from 'Abdullah ibn 'Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Recite the Qur'an in a month." Then I bargained with him, and he bargained with me, until it reached seven days.