Midrash sa Genesis 17:4: Aggadat Bereshit & Bereshit Rabbah

[2] Another interpretation: "And I will establish My covenant" (Genesis 17:2). This refers to the secret of God for those who fear Him, and His covenant to make it known to them (Psalms 25:14). What is the secret of the Holy One, blessed be He? It is a word that the Holy One, blessed be He, did not reveal, a mystery of a word that passed from person to person for twenty generations, until Abraham came and received it, as it is stated, "The secret of the Lord is with those who fear Him, and He will make known to them His covenant" (Psalms 25:14). This refers to Abraham, as it is said of him, "Now I know that you fear God" (Genesis 22:12). God said to him, "If you perform circumcision, you will take hold of the secret, the secret of the Lord for those who fear Him," which is numerically equivalent to seventy (סו"ד בגימטריא שבעים). "Seventy souls" (Deuteronomy 10:22) "And it establishes from them seventy Sanhedrins, as it is said, 'Gather for Me seventy men...' (Numbers 11:16). I establish from them, and I establish from your descendants Moses, who spoke the Torah in seventy languages, as it is stated, "Beyond the Jordan in the land of Moab" (Deuteronomy 1:5). On account of what? On account of circumcision, as it is stated, "The Lord our God made a covenant with us [at Horeb]" (Deuteronomy 5:2). This covenant is circumcision. [edit ref. Pirkei DeRabbi Eliezer 10:10] Therefore, it is said, "The secret of the Lord is with those who fear Him." God said to Abraham, "It is enough for the servant to be like his master." And His covenant, to make it known to them (Psalms 25:14). This can be compared to a king who had a beloved servant who was richer than all others. He said, "What shall I do for this beloved one? What shall I give him? He has gold and silver, he has slaves and maidservants." But then the king said, "Behold, I will take off my belt and give it to him as a sign of my affection." Similarly, God said to Abraham, "What shall I give you? You have gold and silver, as it is said, 'And Abram was very rich' (Genesis 13:2). But what can I give you? I will give you My belt, and establish My covenant between Me and you" (Genesis 17:4). "And Abraham fell on his face"..."Behold, My covenant is with you" ans so on. (Genesis 17:3-4), the secret of the Lord for those who fear Him.

“I, My covenant is hereby with you, and you will be the father of a multitude of nations” (Genesis 17:4).
“I, My covenant is hereby with you” – Rabbi Abba, Rabbi Berekhya, and Rabbi Shmuel bar Ami were sitting and encountering difficulty in determining: From where in the Torah is the use of acronyms derived?³¹From where is it derived that this is an acceptable form of exegesis? It is as it is stated: “The father of a multitude of nations [Av hamon goyim]” – av ham, which is missing the letter reish.³²Abraham is an acronym for av hamon. The reish of “Abraham” is a remnant from his original name, Abram.