It is narrated from Sayyiduna Iyad bin Himar (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever finds a lost item should take two upright witnesses and should not conceal it. If he does not find its owner, then it is the wealth of Allah, which He gives to whomever He wills."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15059