It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "The thorny trees of the Haram are not to be cut, its game is not to be killed, its fresh grass is not to be cut, and it is not permissible to pick up its lost property except for one who will announce it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11267