The son of Sayyiduna Yazid bin al-Aswad al-Suwa'i (may Allah be pleased with him) narrates that his father said: The Prophet (peace and blessings be upon him) led a prayer. When he had completed his prayer, he saw two men standing 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: "What kept you from praying with us?" They said: "O Prophet of Allah, we had prayed in our camp, then we came." The Prophet (peace and blessings be upon him) said: "When you have prayed in your camp and then find the congregational prayer, pray; it will be a voluntary (nafl) prayer for you."
Hadith Referenceالمعجم الكبير للطبراني / باب الياء / 18830