It is narrated from Sayyiduna Yazid bin al-Aswad al-Suwa'i (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) prayed the morning prayer in Masjid al-Khayf. After finishing the prayer, he (peace and blessings be upon him) saw two men in the back part of the mosque. He (peace and blessings be upon him) had them called, and they were brought, trembling. He (peace and blessings be upon him) said: "What is the matter with you, that you did not pray with us?" They said: "We had already prayed in our camp." He (peace and blessings be upon him) said: "When one of you has prayed in his house and then comes to the congregation, let him pray with them as well, for it will be a voluntary (nafl) prayer for him."
Hadith Referenceالمعجم الكبير للطبراني / باب الياء / 18829