Musnad Ahmad 986:2

Al-Walid ibn Muslim narrated to us, Sa'id—meaning Ibn 'Abd al-'Aziz—narrated to us, Makhul narrated to us, from Nu'aym ibn Hammar al-Ghatafani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: Allah the Almighty and Majestic says: "O son of Adam, do not be incapable of four rak'ahs at the beginning of the day, [and] I will suffice you for the end of it."