Khalid bin Duraik (may Allah have mercy on him) said: I asked Ibn Muhayriz (may Allah have mercy on him): "May food and animal fodder be sold in the land of the Romans?" He said: I heard Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) say: "Some people want to make me slip from my religion. That will not happen until death comes to me and I meet Muhammad (peace and blessings be upon him) and his companions. Whoever sells, for gold or silver, food or fodder that he obtained in the land of the Romans, in it is the one-fifth of Allah and the fay' of the Muslims."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18000