Sahih al-Bukhari 91
SahihZayd ibn Khālid al-Juhani (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) was asked about the lost and picked up item that is made of gold or silver, whereupon he said: "Remember the description of its container and the string with which it is tied. Make a public announcement of it for one year. If nobody comes and claims it, then utilize the money but keep it as a trust with you. If its owner comes back one day seeking it, then return it to him." Then, he asked him about the lost camel, whereupon the Prophet (may Allah's peace and blessings be upon him) said: "It is none of your concern. Leave it, for it has its feet and a water container (reservoir), and it will reach water and eat from the trees until its owner finds it." He then asked him about the lost sheep, whereupon he said: "Take it, for it is either for you, or for your brother, or for the wolf."