Imam Maalik مالك بن أنس بن مالك بن أبي عامر · The Book of 'Umra

"Jabir said: 'The Messenger of Allah said: "O Ansar! Hold on to your wealth, and do not give it on the basis of 'Umra. For whoever gives something on the basis of 'Umra, it belongs to the one to whom he gave it on that basis, for the rest of his life and after he dies."'"