Musnad Ahmad 937:36

Marhum ibn Abd al-Aziz al-Attar narrated to us, Abu Imran al-Jawni narrated to me, from Abdullah ibn al-Samit, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Abu Dharr, perform the prayer at its time. If you come to the people and they have already prayed, you will have secured your prayer. And if they have not yet prayed, you will have prayed with them, and it will be a supererogatory (nafilah) prayer for you."