Congregational Prayer and Its Virtue

Ubayd ibn Zurarah reported from Imam Jafar ibn Muhammad Al-Sadiq عليه السلام regarding a man who joined the prayer with the Imam but had missed one rak'ah. After the Imam concluded the prayer, he left with the people and then remembered that he had missed a rak'ah. Imam عليه السلام said: "He should perform one rak'ah”.