Maliks Muwatta Book 36, Hadith Number 48A.
Section : The Judgement on Lost Property.
Malik related to me from Nafi that a man found something and went to Abdullah ibn Umar and said to him, “I have found something. What do you think I should do about it?” Abdullah ibn Umar said to him, “Publicise it!” He said, “I have done so.” He said, “Do it again.” He said, “I have done so.” Abdullah said, “I do not order you to use it. If you wished, you could have left it.”