וַיֹּ֗אמֶר אַל־תִּשְׁלַ֤ח יָֽדְךָ֙ אֶל־הַנַּ֔עַר וְאַל־תַּ֥עַשׂ ל֖וֹ מְא֑וּמָּה כִּ֣י ׀ עַתָּ֣ה יָדַ֗עְתִּי כִּֽי־יְרֵ֤א אֱלֹהִים֙ אַ֔תָּה וְלֹ֥א חָשַׂ֛כְתָּ אֶת־בִּנְךָ֥ אֶת־יְחִידְךָ֖ מִמֶּֽנִּי׃
И сказал он: 'Не возложи на ладонья руку твою и не делай с ним ничего; теперь я знаю, что ты богобоязненный человек, видя, что ты не удержал от Меня сына твоего, единственного твоего сына.'
Rashi
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).
Bekhor Shor
Do not stretch out your hand against the lad: For you have already done the commandment. As He only commanded you to bring him up, and you brought him up over the wood.
Chizkuni
עתה ידעתי, “now I know for a fact, etc;” did G-d not already know? Actually the meaning is: “now I am able to make My knowledge public to all.” We have a similar formulation in Exodus 32,12, where G-d said to Moses: ידעתיך בשם, “I have made your name well known.” The appropriate translation would be: “I have made you so famous that no one can dispute it.”