The Ruling on Lost Property which has not been Announced
Question:
I found a sum of Riyals and I took it and spent it; what should I do now?
Answer:
It is incumbent upon you to announce it, then once you have announced it two or three times a month for a whole year by saying: "To whom do these Dirhams belong which were lost in such-and-such a place?" - in public gathering places and around their doors or in the markets. Once a year has past over your announcements, it is permissible for you to keep it. And if the owner comes and identifies it by its precise description, you must give it to him, because it is the same as a trust with you, so if he comes, you must give it to him. Whoever, if you did not announce it and you used it and kept silent, you must give it in charity in order to exonerate yourself, with the intention of giving it on behalf of its owner, because you did not do the thing necessary to make it lawful to you - which is to announce it - then he will receive the reward of it, by Allaah's Permission - the Almighty, the All-Powerful - And Allaah is the Granter of success.
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah, volume 5 / page 24-25
