It is allowed to pray tarawih of the month of Ramadan in a congregation just as it is allowed to pray them on an individual basis. The majority of the scholars, however, prefer to pray them in congregation. The Prophet sallallahu alehi wasallam, as stated earlier, prayed tarawih in congregation with the Muslims but he discontinued since he feared that it would be made obligatory.
‘Umar was the one who convoked the Muslims to pray tarawih behind one imam. Abdurahman ibn Abdulqari reports: “One night during Ramadan, I went with ‘Umar to the mosque and the people were praying in different groups. Some were praying by themselves and others were praying in small groups. ‘Umar said: ‘I think it would be better if I gathered them under one imam.’ Then he did so and appointed Ubayy ibn Ka’b as the leader of the prayer. Then I went out with him on another night and all the people were praying behind one imam and ‘Umar said: ‘What a good innovation (bid’ah) this is,’ but, it is better to sleep and delay it until the latter portion of the night.” The people (however) prayed it at the beginning of the night. This is related by al-Bukhari, Ibn Khuzaimah, alBaihaqi, and others.