Salim bin 'Abdullah bin 'Umar سالم بن عبد الله بن عمر بن الخطاب · The Book on Zakat

"The Messenger of Allah had a letter written about charity, but he had not dispatched it to his governors until he died; he kept it with him along with his sword. When he died, Abu Bakr implemented it until he died, as did Umar until he died. In it was: 'A sheep (is due) on five camels, two sheeps on ten, three sheeps on fifteen, four sheeps for twenty, a Bint Makhad on twenty-five to thirty-five. When it is more…

the Messenger of Allah instituted the Ushr of what was watered by the heavens and steams, or through natural channels, and half of the Ushr for what is watered by irrigation.

Umar gave a horse to be used in the cause of Allah. Then he saw it being sold, so he wanted to buy it, but the Prophet said: "Do not take back what you have given in charity."