Sahih Muslim Book 5, Hadith Number 2243.

Chapter : Exhortation to give charity even though it is small; (the one who gives) small amount should neither be discouraged not looked down upon.

Asma’, daughter of Abu Bakr (Allah be pleated with him), reported: The Messenger of Allah (may peace be upon him) said to me: Spend, and do not calculate, or otherwise Allah would also calculate in your case.