Ruth 4:11,12

And all the people that were in the gate, and the elders, said, We are witnesses. The LORD make the woman that is come into thine house like Rachel and like Leah, which two did build the house of Israel: and do thou worthily in Ephratah, and be famous in Bethlehem: (12) And let thy house be like the house of Pharez, whom Tamar bare unto Judah, of the seed which the LORD shall give thee of this young woman.

It is easy to gather wherefore, in this blessing, the wives of the Patriarch Jacob are mentioned rather than those of Abraham or Isaac. Sarah had but Isaac in the covenant; and Rebekah but a Jacob. Her other son Esau rejected the blessing. Whereas Rachel, and Leah, were mothers to several' of the tribes of Israel. Pharez, who descended from Judah, was the progenitor of the Bethlehemite.