Genesis 16:3

And Sarai Abram's wife took Hagar her maid the Egyptian, after Abram had dwelt ten years in the land of Canaan, and gave her to her husband Abram to be his wife.

Sarai ... took Hagar ... and gave her to her husband Abram to be his wife. Wife is here used to describe an inferior, though not degrading relation, in countries where polygamy prevails. In the case of these female slaves, which are the personal property of his lady, being purchased before her marriage, or given as a special present to herself, no one can become the husband's secondary wife without her mistress' consent or permission. This usage seems to have prevailed in patriarchal times; and Hagar, the slave of Sarai, of whom she had the entire right of disposing, was given by her mistress' spontaneous offer, to be the secondary wife of Abram, in the hope of obtaining the long-looked for heir. It was a temporary conversation with a bondwoman with a special object in view; and it ceased as soon as Hagar had conceived.

A similar usage still exists in many parts of the East, where childless wives, being liable, conformably with law or custom, to be divorced, have naturally resorted to a scheme which prevents repudiation. When charms, incantations, and pilgrimages to sacred shrines have failed to render them productive, the wives of Oriental grandees, particularly Hindus and Musselmen, very commonly appropriate their own maid-servants to their husbands for the purpose of procuring an heir; and being acceded to on their part for the attainment of that specific object, it does not alienate their affections from their lawful partners. The son, born of the bondwoman, and nursed by her, is called the child of the lady of the house, and is treated as such by all the friends and visitors of the family. In the patriarch's case, however, this extemporaneous connection, though, according to the usages of the East, not dishonourable or immoral, was a wrong step: it betrayed a want of faith and simple reliance on God; and the issue, as the apostle informs us, was not legitimate, or entitled to inherit the property of Abram (Galatians 4:30).