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.