Sayyiduna al-Jarud (may Allah be pleased with him) narrated: We were with the Messenger of Allah (peace and blessings be upon him) on a journey, and riding camels were scarce, so we discussed among ourselves what would suffice us. We said: There are some camels we pass by on a riverbank, so we make use of their backs. The Messenger of Allah (peace and blessings be upon him) said: "The lost property of a Muslim is a burning of the Fire," saying it three times, "so do not go near it."
And the Prophet of Allah (peace and blessings be upon him) said: "Regarding a lost item that you find, do not hide it away and do not conceal it. If you recognise the owner, give it to him; otherwise it is the wealth of Allah, who gives it to whomever He wills."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2109