Bishr bin Bashir al-Aslami narrates from his father, Sayyiduna Bashir al-Aslami (may Allah be pleased with him), who said: When the Muhajirun came to Madinah, they found the water (there) disagreeable. A man from Banu Ghifar had a spring called Rumah, and he used to sell a waterskin of it for a mudd. The Messenger of Allah (peace and blessings be upon him) said to him: "Sell it to me in exchange for a spring in Paradise." He said: O Messenger of Allah, I and my family have nothing besides it; I cannot do that. This reached Sayyiduna Uthman (may Allah be pleased with him), so he bought it for thirty-five thousand dirhams. Then he came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah, if I buy it, will you grant me the same as what you offered him, a spring in Paradise? He (peace and blessings be upon him) said: "Yes." He said: I have bought it, and I have dedicated it to the Muslims.
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1215