Silsilat al-Ahadith al-Sahihah

Sayyiduna Jarud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The lost property of a Muslim is a flame of fire."

Sayyiduna Abu Umamah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "A borrowed item must be returned, and a goat given temporarily for its milk must also be returned. Whoever picks up a lost item that is tied with a string or something similar, it is not lawful for him to untie it until he shows it to someone else (making him a witness)."