Musnad Ahmad 1211:2

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah informed us, from Yazid ibn Abi Habib, from al-Harith ibn Ya'qub, from Ya'qub ibn al-Ashajj, from 'Amir ibn Sa'd, from Sa'd, from Khawlah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever descends at a stopping place and says: 'I seek refuge in the perfect words of Allah from the evil of what He has created,' nothing in it shall harm him until he departs from it."