Musnad Ahmad 487:2

'Abd al-Rahman ibn Mahdi narrated to us, Sufyan narrated to us, from Ya'la ibn 'Ata', from Jabir ibn Yazid ibn al-Aswad, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) prayed the Fajr prayer in Mina. He then turned and saw two men behind the people, so he called for them. They were brought, and their shoulder muscles were trembling. He said: "What prevented you from praying with the people?" They said: "We had already prayed in our dwellings." He said: "Do not do that. If one of you prays in his dwelling and then catches the prayer with the Imam, let him pray it with him, for it is a supererogatory (nafilah) prayer for him."