Musannaf Ibn Abi Shaybah 5412 — Speaking when the imam has ascended

Sayyiduna Jabir (may Allah be pleased with him) said: Once, on a Friday, Sayyiduna Sa'd (may Allah be pleased with him) said to a man: "Your prayer did not count." The man mentioned this to the Prophet (peace and blessings be upon him) and said: "Sa'd told me that my prayer did not count." The Prophet (peace and blessings be upon him) asked Sa'd: "Why did you say that?" He replied: "He talked while you were delivering the sermon." The Prophet (peace and blessings be upon him) said: "Sa'd is right."