Hani' bin Kulthum (may Allah have mercy on him) said: After the conquest of Syria, the commander of the army wrote to Umar (may Allah be pleased with him): "We have conquered a land abundant in food and animal fodder. I do not want to take any step without your order, so write instructions to us." So Sayyiduna Umar (may Allah be pleased with him) wrote: "Do not stop the people from eating and obtaining fodder, and whatever is sold for gold or silver, in it is the one-fifth and the share of the Muslims."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18002