Ata bin Abi Rabah narrates this statement from Sayyiduna Jabir bin Abdullah (may Allah be pleased with him): "It is the established practice that when there are three people present, then one of them will be the imam, and wherever there are forty people or more, then there will also be the Friday prayer, the Eid al-Adha prayer, and the Eid al-Fitr prayer. The reason for this is: these people are considered a congregation."