ANSWER: The Lutheran Study Bible lists five occasions where Genesis 15:6 is quoted in the New Testament: Paul uses it three times in the fourth chapter of Romans (vv. 3, 9, 22) and once in Galatians (3:6), and James uses it once (2:23).
Note: In Luther’s Small Catechism Genesis 15:6 is the first of three verses used to support the answer the question, “How do you receive this forgiveness of sins?”. The two other verses are Romans 3:28 and Romans 4:5.
The answer to the catechism question is, “I receive this forgiveness through faith, that is, by believing the Gospel.”