Musannaf Ibn Abi Shayba 13:1233

Abu Bakr narrated to us, he said: Isma'il bin 'Ulayyah and Ibn Abi Za'idah narrated to us, from Ibn 'Awn, from al-Qasim, who said: "Were it not that Ibn 'Umar disliked the *thunya* (exception/reservation of a portion of the produce), and he was agreeable to us, we would not have seen any harm in that." Ibn 'Ulayyah added, from Ibn 'Awn: "So we spoke about how Ibn 'Umar used to say: 'I will not sell this palm tree, nor this palm tree.'"