It is narrated from Hazrat Ubayy bin Ka'b (may Allah be pleased with him) that once on a Friday, the Noble Prophet (peace and blessings be upon him) recited Surah Bara'ah from the pulpit. At that time, the Noble Prophet (peace and blessings be upon him) was standing and mentioning the favors of Allah. Hazrat Ubayy bin Ka'b (may Allah be pleased with him) was sitting in front of the Noble Prophet (peace and blessings be upon him), and with him were Hazrat Abu Darda (may Allah be pleased with him) and Abu Dharr Ghifari (may Allah be pleased with him) also sitting. One of them pinched Hazrat Ubayy (may Allah be pleased with him) and said, "Ubayy! When was this Surah revealed? I am hearing it just now." Hazrat Ubayy bin Ka'b (may Allah be pleased with him) signaled him to remain silent. After the prayer was over, he said, "I asked you when this Surah was revealed, so why did you not tell me?" Hazrat Ubayy (may Allah be pleased with him) replied, "Today your prayer is only as much as the vain act you did in it." He went to the Prophet (peace and blessings be upon him) and mentioned what Hazrat Ubayy (may Allah be pleased with him) had said. The Prophet (peace and blessings be upon him) said, "Ubayy has spoken the truth. When you see the Imam speaking, remain silent until he finishes."