In Romans 4:3, who 'believed God, and it was credited to him as righteousness'?

Answer: Abraham

Paul points to Abraham: "Abraham believed God, and it was credited to him as righteousness" (KJV: counted unto him).

“What then shall we say that Abraham, our forefather according to the flesh, has discovered? If Abraham was indeed justified by works, he had something to boast about, but not before God. For what does the Scripture say? “Abraham believed God, and it was credited to him as righteousness.””— Romans 4:1-3 (BSB)