Musannaf Ibn Abi Shayba 13:1539

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from his father, from ‘Amir, from Shurayh, who said: ‘Umar made a covenant with me that I should not validate a gift of a kingdom (or property) until it has remained in its house for a year, or it has given birth to a generation.