Why Abraham and Jacob Swore Oaths Upon Circumcision

He said to him (Genesis 24:2): "Place now your hand [under my thigh]." See how they cherished circumcision! And Jacob too, when he made Joseph swear, said to him (Genesis 47:29): "Place [now your hand under my thigh]." For what reason did they cherish circumcision? Because they knew that it was destined to deliver their children from Gehinnom in the world to come, as it is said (Isaiah 5:14): "Therefore Sheol has enlarged its appetite [and opened its mouth without a statute]." What is the meaning of "without a statute"? There is no "statute" but circumcision, as it is said (Psalms 105:9-10): "which He cut with Abraham..." and "He established it for Jacob as a statute." But Israel, who are circumcised, are delivered from it, as it is said (Isaiah 43:2): "When you pass through the waters [I will be with you]."