Musannaf Ibn Abi Shayba 37:331

37439. Malik ibn Isma'il narrated to us, from Sharik, from 'Uthman ibn Abi Zur'ah, from Salih, from 'Ali, who said: "Do not cultivate with me in the Sawad (the fertile lands of Iraq), for if you cultivate, you will fight one another over the water with swords. And if you fight one another, you will commit kufr (disbelief)."