Muhammad bin 'Amr bin 'Alqama محمد بن عمرو بن علقمة · The Book of 'Umra

"I heard the Messenger of Allah say: 'Whoever gives a life-long gift to a man, it belongs to him and to his heirs; his words (when he gave the gift) put an end to his rights over it, and it belongs to the one to whom it was given on the basis of 'Umra, and to his heirs.'"

"A lifelong gift ('Umra) is permissible."