Because it is contained in Scripture, "Behold, I lay in Zion a chief cornerstone, chosen, and precious: He who believes in him will not be disappointed."
A stone of stumbling, and a rock of offence ...  Some have been puzzled by Peter's putting these two passages from the Psalms and Isaiah together, exactly as Paul did in Romans, and have therefore supposed Peter's dependence on Paul; but such a device is both erroneous and unnecessary. Peter was present no doubt and heard the Lord Jesus Christ himself put the two passages together in exactly the same manner as here (Luke 20:17,18). Therefore, neither Peter nor Paul was dependent upon the other, their teachings, as in the case of all the sacred writers, going back to Christ himself, the fountain source of the entire New Testament.