Musnad Ahmad 32:657

Haytham ibn Kharijah narrated to us, Rashdin ibn Sa’d narrated to us, from Abdullah ibn al-Walid, from Abu Hafs, that he narrated to him that he heard Anas ibn Malik saying: The Prophet, may Allah bless him and grant him peace, said: "Indeed, the likeness of the scholars on the earth is like the stars in the sky, by which one is guided in the darkness of the land and the sea. So if the stars are extinguished, the guides are about to go astray."