It is narrated from Habban bin al-Harith: I came to Ali (may Allah be pleased with him) while he was encamped at Dayr Abi Musa, and he was eating. He said to me: Eat. I said: I intend to fast. He said: I too intend to fast. So I came closer and began to eat. When he had finished, he said: O Ibn al-Tayyah, call the iqamah for the prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2147