It is narrated from al-Sha'bi (may Allah have mercy on him) that a man came to Sayyiduna Ali (may Allah be pleased with him) and said: I found fifteen hundred dirhams in a ruin! Sayyiduna Ali (may Allah be pleased with him) said: I shall give a clear judgement on it. If you found it in a village whose tax is paid by another village, then it belongs to them; and if you found it in a place whose land tax (kharaj) is not paid by any village, then four parts of it are yours and the fifth is ours; then the fifth is also yours.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7655