It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that a man began to pray alone, so the Messenger of Allah (peace and blessings be upon him) said: "Will no man give charity to this one?" So a man stood up and joined him in the prayer, and the Messenger of Allah (peace and blessings be upon him) said: "These two are a congregation."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7826