It is narrated from Sayyiduna Zayd bin Thabit (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever gives something as an umra, it belongs to the one to whom it was given, both during his life and after his death. Do not make the condition of ruqba; whoever makes the condition of ruqba on anything, it will follow the same path (of becoming his property)."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4925