Sahih Muslim Book 5, Hadith Number 2358.

Chapter : To please the collector of Zakat, unless he makes an unjust demand.

‘Abdullah b. Abu Aufa said that it was the common practice of the Messenger of Allah (may peace be upon him) that when the people brought to him sadaqa he blessed them: O Allah, bless them. So when Abu Aufa brought to him Sadaqa he (the Holy Prophet) said: O Allah, bless the posterity of Abu Aufa.