The son of Sayyiduna Yazid bin al-Aswad al-Suwa'i (may Allah be pleased with him) narrates that his father said: We prayed the Fajr prayer with the Messenger of Allah (peace and blessings be upon him). Then he (peace and blessings be upon him) turned around and saw two men behind the people. He (peace and blessings be upon him) had them called, and they were brought, trembling. He (peace and blessings be upon him) said: "Why did you two not pray with the people?" They said: "O Messenger of Allah, we had already prayed in our camp." He (peace and blessings be upon him) said: "Do not do that. When one of you has prayed in his camp and then finds the prayer with the imam, let him pray it with him as well, for it will be a voluntary (nafl) prayer for him."
Hadith Referenceالمعجم الكبير للطبراني / باب الياء / 18828