Musannaf Ibn Abi Shayba 18:641

Abu Bakr narrated to us, he said: Abu Usamah narrated to us, from Isma'il, from al-Harith, from Shurayh: That he would not leave a canopy (or roof) that a horseman could not pass under with his spear, and he would say: "You have built upon the spear of the horseman."