Where in the Bible does it say that believers are the “seed of Abraham”?

In Galatians 3:29 (in the ESV) it says:

…if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.

The word offspring is sometimes translated as seed (e.g., in the HCSB, NIrV, TNIV, NKJV, KJV) or descendants (e.g., CEB, GNB, LEB, NASB, NET). They all mean the same thing. Paul is saying that if we are involved with Jesus then that puts us in the group who will share in the promises that God made to Abraham.

Further reading

For more on God’s promises to Abraham, see the following PDF: God’s Promises to Abraham