Musnad Ahmad 482:99

Yazid ibn Harun narrated to us, Aban ibn Yazid al-Attar narrated to us, from Qatadah, from Nu'aym ibn Hammar, from Uqbah ibn Amir al-Juhani, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah says: 'O son of Adam, suffice Me [with] four rak'ahs at the beginning of the day, and I will suffice you with them at the end of your day.'"