Musannaf Ibn Abi Shayba 3:5222

Ibn Idris narrated to us, from Shu'bah, from Abu 'Imran al-Jawni, from 'Abdullah ibn al-Samit, from Abu Dharr, who said: "My friend (the Prophet) advised me to perform the prayer at its time. If you reach the people and they have already prayed, you have secured your prayer; otherwise, it will be a supererogatory (nafilah) prayer."