Musnad Ahmad 986:5

Yahya ibn Ishaq narrated to us, Sa'id ibn 'Abd al-'Aziz informed me, from Makhul, from Ibn Murrah al-Ta'ifi, who said: I heard the Prophet, may Allah bless him and grant him peace, saying: Allah, the Mighty and Majestic, says: "O son of Adam, pray four rak'ahs for Me at the beginning of the day, and I will suffice you for the end of it."