Musnad Ahmad 986:1

'Abd al-Rahman ibn Mahdi narrated to us, Mu'awiyah—meaning ibn Salih—narrated to us, from Abu al-Zahiriyah, from Kathir ibn Murrah, from Nu'aym ibn Hammar al-Ghatafani, that he heard the Messenger of Allah (may Allah bless him and grant him peace) say: 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."