Musnad Ahmad 509:18

Yazid ibn Harun narrated to us, Isra'il ibn Yunus informed us, 'Umar ibn 'Abdullah ibn Ya'la narrated to me, from his grandmother Hakima, from her father Ya'la—Yazid said: in what Ya'la ibn Murrah narrates—he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever finds a minor lost item, a dirham or a rope or something similar to that, let him announce it for three days. If it is more than that, let him announce it for six days."