Mishkat al-Masabih 486

Yahya b. Abd ar-Rahman said that ‘Umar went out with a party of riders among whom was ‘Amr b al-‘As. When they came to a cistern and ‘Amr asked its owner whether beasts of prey came down to it, ‘Umar said, “Don't tell us, owner of the cistern, for we go down to what beasts of prey leave, and they go down to what we leave.” Malik transmitted it and Razin made an addition saying that some transmitters add to ‘Umar’s words that he said he had heard God’s messenger say, “They have in their bellies what they have taken, and what remains is pure for us and drinkable.”