Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings be upon him) said: "Whichever man is given something as a lifelong grant ('umra) for him and his heirs, and (the giver) said: 'I have given it to you and your heirs as long as any one of you remains,' then it belongs to the one to whom it was given, and it does not return to its original owner, because he has given a gift in which inheritance has come into effect." In one narration the words are: "in which inheritance comes into effect."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الهبات / 11963