Muwatta Malik, Book 36, Hadith 47 (USC-MSA)

Malik related to me from Ayyub ibn Musa from Muawiya ibn Abdullah ibn Badr al-Juhani that his father informed him that he stopped with a people on the way to Syria and he found a purse which had eighty dinars in it. He mentioned that to Umar ibn al-Khattab. Umar said to him, "Announce it at the doors of the mosques and mention it to everyone who comes from Syria for a year. When a year passes, it is your business."

وَحَدَّثَنِي مَالِكٌ، عَنْ أَيُّوبَ بْنِ مُوسَى، عَنْ مُعَاوِيَةَ بْنِ عَبْدِ اللَّهِ بْنِ بَدْرٍ الْجُهَنِيِّ، أَنَّ أَبَاهُ، أَخْبَرَهُ أَنَّهُ، نَزَلَ مَنْزِلَ قَوْمٍ بِطَرِيقِ الشَّامِ فَوَجَدَ صُرَّةً فِيهَا ثَمَانُونَ دِينَارًا فَذَكَرَهَا لِعُمَرَ بْنِ الْخَطَّابِ فَقَالَ لَهُ عُمَرُ عَرِّفْهَا عَلَى أَبْوَابِ الْمَسَاجِدِ وَاذْكُرْهَا لِكُلِّ مَنْ يَأْتِي مِنَ الشَّأْمِ سَنَةً فَإِذَا مَضَتِ السَّنَةُ فَشَأْنَكَ بِهَا ‏.‏

sunnah.com

also referenced under: Book 36, Hadith 1450 (In-Book)