One-fifth of Booty to the Cause of Allah (Khumus)
Bukhari :: Book 4 :: Volume 53 :: Hadith 357
Narrated Anas bin Malik:
People used to give some of their datepalms to the Prophet (as a gift), till he conquered Bani Quraiza and Bani An-Nadir, whereupon he started returning their favors.
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