Sahih Al Bukhari – Book of Obligatory Charity Tax (Zakat)

Volumn 002, Book 024, Hadith Number 498.
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Narated By 'Adi bin Hatim RA: Heard the Prophet PBUH saying: "Save yourself from Hell-fire even by giving half a date-fruit in charity."

Comment: this Hadith encourages us to give charity even very small amounts (5 rupees a day or small food to a poor etc).