Romans 4:21

American Standard Version

and being fully assured that what he had promised, he was able also to perform.

Romans 4:21 in other translations

and being fully assured that what he had promised, he was also able to perform.

19And without being weakened in faith he considered his own body now as good as dead (he being about a hundred years old), and the deadness of Sarah’s womb;
20yet, looking unto the promise of God, he wavered not through unbelief, but waxed strong through faith, giving glory to God,
21and being fully assured that what he had promised, he was able also to perform.
22Wherefore also it was reckoned unto him for righteousness.
23Now it was not written for his sake alone, that it was reckoned unto him;