Genesis 30:4

And she gave him Bilhah her handmaid
To be enjoyed as a wife, though she was no other than a concubine; yet such were sometimes called wives, and were secondary ones, and were under the proper lawful wife, nor did their children inherit; but those which Jacob had by his wives' maids did inherit with the rest: and Jacob went in unto her;
consenting to what Rachel his wife proposed to him: having concubines, as well as more wives than one, were not thought criminal in those times, and were suffered of God, and in this case for the multiplication of Jacob's seed; and perhaps he might the more readily comply with the motion of his wife, from the example of his grandfather Abraham, who took Hagar to wife at the instance of Sarah.