Chazal tells us (Toldos 26Midrash Rabbah Bereishis 64:5;4 and 95:3; Yoma 28b, based on. Gen. 26:5; Kiddushin 82a) that the Patriarchs observed the mitzvahs before they were given. But Avraham did not circumcize himself or his male household members until specifically commanded. Why did he wait?