מדרש אויף ויקרא 8:23

21) (Vayikra 8:23): "And he slaughtered, and Moses took of its blood": The three bloods mentioned in this section are not similar one to the other (see Vayikra 8:33 below). The first requires six applications; the second, two; and the third, thirteen. This is the intent of "And he slaughtered and he took its (particular) blood." "and he placed it on the tnuch of Aaron's right ear": This (tnuch) is the middle wall. "and upon the thumb of his right hand": This is the middle joint.