Sahih Muslim Book 18, Hadith Number 4274.

This hadith has been narrated on the authority of Rabi’a b. Abu Abd al-Rahman with the same chain of transmitters but with this addition: “There came a person to Allah’s Messenger (may peace be upon him) while I was with him, and he asked him about picking up of a stray article, and he said: When none comes to demand it, then spend that.”