Musnad Ahmad 33:142

Hushaym narrated to us, Dawud informed us, from Abu al-Zubayr, from Jabir, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Al-'Umra (a gift of a house for life) is permissible for its people, and al-Ruqba (a gift contingent upon who dies first) is permissible for its people."