Musnad Ahmad 938:51

'Abd al-Rahman narrated to us, Sufyan narrated to us—he dictated it to us—from Ibn Abi Najih, from Tawus, from a man, from Zayd ibn Thabit: "That the Messenger of Allah (peace and blessings of Allah be upon him) assigned the *ruqba* (a gift contingent upon the death of one of the two parties) to the heir."