Sayyiduna al-Jarud (may Allah be pleased with him) narrated: We were with the Prophet (peace and blessings be upon him) on a journey, and riding camels were scarce, so the people discussed riding mounts among themselves. I said: O Messenger of Allah, we pass by camels grazing on a riverbank; may we benefit from their backs? He (peace and blessings be upon him) said: "No, the lost property of a Muslim is a burning of the Fire; the lost property of a Muslim is a burning of the Fire; the lost property of a Muslim is a burning of the Fire," saying it three times. Then he said: "And as for a dropped item that you find, announce it, then do not conceal it and do not hide it away. If you find its owner, give it to him; otherwise it is the wealth of Allah, the Mighty and Majestic, who gives it to whomever He wills."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2107