(a) Abu Ubayd (may Allah have mercy on him) says: This is how I memorised it from him; then later it reached me that he had no doubt about it: "When your imam asks you for food, feed him."
(b) Abu Abd al-Rahman (may Allah have mercy on him) narrates it like the first saying, but with an addition: What is his asking for food? He said: "When he falters and falls silent, prompt him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5792