When Joseph appeared in the company of his two sons, his father said to him: "In all the seventeen years thou hast been visiting me, thou didst never bring thy sons with thee, but now they have come, and I know the reason. If I bless them, I shall act in opposition to the word of God, who promised to make me the progenitor of twelve tribes, for if I adopt them as my sons, there will be fourteen tribes.
But if I do not bless them, it will plunge thee in sorrow. So be it, I will bless them. But think not I do it because thou didst support me all these years. There is quite another reason.
When I left my father's house to go to Haran, I offered up a prayer at Beth-el, and I promised to give unto God the tenth of all I owned. So far as my material possessions are concerned, I kept my vow, but I could not give the tithe of my sons, because according to the law I had to withdraw from the reckoning the four sons, Reuben, Joseph, Dan, and Gad, that are the first-born children of their mothers.
When I returned, God again appeared unto me in Beth-el, and He said, Be fruitful and multiply. But after this blessing no son was born unto me except Benjamin alone, and it cannot be but that God meant Manasseh and Ephraim when He spoke of 'a nation and a company of nations.' If now I have found favor in thy sight, thy two sons Ephraim and Manasseh, even as Reuben and Simon, shall be mine, and then I shall be able to give a tenth part of my ten sons unto the Lord, and I shall leave this world free from the sin of not keeping my vow to the Lord concerning the tithe-giving."