Where is the Scripture that states Abraham’s strength did not diminish?

Answer:

I believe you are confusing Abraham with Moses.

Regarding Abraham's life, the Bible says, "This is the sum of the years of Abraham's life which he lived: one hundred and seventy-five years. Then Abraham breathed his last and died in a good old age, an old man and full of years, and was gathered to his people" (Genesis 25:7-8).

Regarding Moses' life, we read, "Moses was one hundred and twenty years old when he died. His eyes were not dim nor his natural vigor diminished" (Deuteronomy 34:7).

The only other which mentions remaining strong into old age was Caleb. "And now, behold, the LORD has kept me alive, as He said, these forty-five years, ever since the LORD spoke this word to Moses while Israel wandered in the wilderness; and now, here I am this day, eighty-five years old. As yet I am as strong this day as on the day that Moses sent me; just as my strength was then, so now is my strength for war, both for going out and for coming in" (Joshua 14:10-11).