Musnad Abu Ya'la al-Mawsili 109:215

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Dawud bin Abi Hind informed us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah (peace be upon him) said: "The *'umra* (gift of a house for life) is permissible for the one to whom it is given. And the *ruqba* (gift of a house contingent upon who dies first) is permissible for the one to whom it is given."