Sahih Bukhari
One-fifth of Booty to the Cause of Allah (Khumus)
Volume: 004 / Book Number: 053 / Hadith Number: 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.