Musnad Ahmad 986:6

Hammad ibn Khalid narrated to us, Mu'awiyah narrated to us, from Abu al-Zahiriyah, from Kathir ibn Murrah, from Nu'aym ibn Hammar, who said: I heard the Prophet, may Allah bless him and grant him peace, saying: Allah, the Almighty and Majestic, said: "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."