Musnad Abu Ya'la al-Mawsili 153:103

Ali ibn al-Ja'd narrated to us, Ibn Abi Dhi'b informed us, from Ibn Shihab, from Salim, from Ibn Umar, who said: The Messenger of Allah (peace be upon him) said: "Whoever buys a palm tree after it has been pollinated, and does not stipulate its fruit, then he has no right to it. And whoever buys a slave and does not stipulate his wealth, then he has no right to it."