Silence During the Friday Sermon
On one occasion, the Holy Prophet (sa) was delivering the Friday Sermon and recited Surah Bara‘at. Hazrat Abu Darda (ra) and Hazrat Abu Dharr (ra) did not yet know of this chapter. So during the sermon, they indicated through a gesture to Hazrat Ubayy (ra) in order to ask when this chapter was revealed as they had not yet heard it. Hazrat Ubayy (ra) indicated that they should remain silent.
Upon the completion of prayer, when Hazrat Ubayy (ra) stood up to go home, both of these noble men asked Hazrat Ubayy (ra) why he had not answered their question. Hazrat Ubayy (ra) replied that their prayer that day had become invalidated owing to a vain act. Hearing this, they both went to the Holy Prophet (sa) and related to him what Hazrat Ubayy (ra) had said. The Holy Prophet (sa) stated that indeed Hazrat Ubayy (ra) had spoken the truth; in other words, they should not have spoken whilst the [Friday] sermon was being delivered.
(Sheikh Shah Moinuddin Ahmad Nadvi, Siyar al-Sahabah, Vol. 3 [Karachi, Pakistan: Dar al-Ishaah Urdu Bazar, 2004] 157)