Bereshit 22:12

אל תשלח LAY NOT THINE HAND [UPON THE LAD] to slay him. Then he (Abraham) said to God, “If this be so, I have come here for nothing; let me at least inflict a wound on him and draw some blood from him”. God replied, “Neither do thou anything (מאומה) to him” — inflict no blemish (מום) on him (Genesis Rabbah 56:7). כי עתה ידעתי FOR NOW I KNOW—R. Aba said: Abraham said to God, “I will lay my complaint before you. Yesterday (on an earlier occasion) you told me, (Genesis 21:12) “In Isaac shall seed be called to thee”, and then again you said, (Genesis 21:2) “Take now thy son”. Now you tell me, “Lay not thy hand upon the lad”! The Holy One, blessed be He, said to him, in the words of Psalms 89:35, “My covenant will I not profane, nor alter that which is gone out of My lips”. When I told you, “Take thy son”, I was not altering that which went out from My lips, namely, My promise that you would have descendants through Isaac. I did not tell you “Slay him” but bring him up to the mountain. You have brought him up — take him down again” (Genesis Rabbah 56:8). כי עתה ידעתי FOR NOW I KNOW — From now I have a reply to give to Satan and to the nations who wonder at the love I bear you: I have an opening of the mouth (i.e. I have an excuse, a reason to give them) now that they see that you are a God-fearing man (Genesis Rabbah 56:7).

וַיֹּ֗אמֶר אַל־תִּשְׁלַ֤ח יָֽדְךָ֙ אֶל־הַנַּ֔עַר וְאַל־תַּ֥עַשׂ ל֖וֹ מְא֑וּמָה כִּ֣י ׀ עַתָּ֣ה יָדַ֗עְתִּי כִּֽי־יְרֵ֤א אֱלֹהִים֙ אַ֔תָּה וְלֹ֥א חָשַׂ֛כְתָּ אֶת־בִּנְךָ֥ אֶת־יְחִידְךָ֖ מִמֶּֽנִּי׃

And he said, Lay not thy hand upon the lad, neither do anything to him: for now I know that thou fearest God, seeing thou hast not withheld thy son, thy only son from me.