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said—this is Yahweh's declaration
"spoke this message from Yahweh" or "declared these words of Yahweh." This is a formal way of saying that the words that follow come directly from Yahweh.
by myself I have sworn
"I have promised and I am my witness." To swear means to use the name of something or someone as the basis or power on which the oath is made. There is nothing more powerful for Yahweh to swear by than himself.
you have done this thing
"you obeyed me"
have not withheld your son
"have not kept back your son." This can be stated in positive form. AT: "were willing to offer your son to me" (See: rc://en/ta/man/translate/figs-doublenegatives)
your son, your only son
It is implied that God knows that Abraham has another son, Ishmael. This emphasizes that Isaac is the son that God promised to give Abraham. See how you translated a similar phrase in Genesis 22:2. (See: rc://en/ta/man/translate/figs-explicit)