Abu Abd al-Rahman (may Allah have mercy on him) narrates from Sayyiduna Ali (may Allah be pleased with him) that it is the sunnah that "when the imam asks you for a prompt, you prompt him." I said: Abu Abd al-Rahman! What is meant by the imam asking for food? He said: When he falls silent.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5793