BT Šab 149a(29) :אילימ׳ דכתי׳ ומתתאי ליחוש שמא ימחוק אלא לאו דכתי׳ אכותל ומדלאי‏ : should we say it is written down below, then one would worry lest he erase it, but not if it is written on a wall and above
See the entire entry for mdlˀy adv. from above