Musnad Abu Ya'la al-Mawsili 109:93

Hajjaj narrated to us, Yazid bin Harun narrated to us, Ibn Abi Dhi'b informed us, from al-Zuhri, from Abu Salamah, from Jabir, from the Prophet (peace and blessings of Allah be upon him), who said: "Whoever is granted a life-gift (umra) for himself and his descendants, it belongs to him permanently. It is not permissible for the giver to impose a condition upon it, nor a reversion."