"And they shall place it on the two side posts and on the lintel": I might think that if he placed (the blood on) one before the other, he has not fulfilled his obligation. It is, therefore, written (Ibid. 22) "and you shall touch the lintel and the two doorposts," (to teach that even) if he reversed the order, he has fulfilled his obligation. We hereby learn that our forefathers in Egypt had three altars—the lintel and two door-posts. R. Shimon says: Four: the saf, the lintel, and two door-posts.