Sahih Muslim Book 5, Hadith Number 2189.

Chapter : Getting of reward to Sadaqa for the dead given on his or her behalf.

A Hadith like this has been narrated on the authority of Zainab the wife of ‘Abdullah, and she said: I was in the mosque and the Prophet of Allah (may peace be upon him) saw me and said: Give Sadaqa even though it is out of your jewellery. The rest of the Hadith is the same.