al-Nadr bin Anas bin Malik النضر بن أنس بن مالك الأنصاري · The Book of 'Umra

"Any man who is given a gift on the basis of 'Umra, it belongs to him and to his descendants. It belongs to the one to whom he gave it, and it cannot be taken back by the one who gave it, because he has given a gift, and it comes to the heirs of the one to whom it was given."