Sahih Muslim 2913aBook 54, Hadith 81
"We were in the company of Jabir b. 'Abdullah that he said it may happen that the people of Iraq may not send their qafiz and dirhams (their measures of food stuff and their money). We said: Who would be responsible for it? He said: The non-Arabs would prevent them. He again said: There is the possibility that the people of Syria may not send their dinars and mudds. We said: Who would be responsible for it? He said this prevention would be made by the Romans. He (Jabir b. Abdullah) kept quiet for a while and then reported Allah's Messenger (ﷺ) having said there would be a caliph in the last (period) of my Ummah who would freely give handfuls of wealth to the people without counting it. I said to Abu Nadra and Abu al-'Ala: Do you mean 'Umar b. 'Abd al-Aziz? They said: No (he would be Imam Mahdi).