Musnad Ahmad 31:283

Waki' narrated to us, Sharik narrated to us, from Yazid ibn Abi Ziyad, from Ibn Abi Nu'm, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about a person in the state of ihram killing a snake. He said: "There is no harm in it."