Who told Abram (Abraham) to have sex with his wife's slave girl?

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Sarai (Sarah, Abraham's wife)

Now Sarai Abram's wife bare him no children: and she had an handmaid, an Egyptian, whose name was Hagar.

And Sarai said unto Abram, Behold now, the LORD hath restrained me from bearing: I pray thee, go in unto my maid; it may be that I may obtain children by her. And Abram hearkened to the voice of Sarai. Genesis 16:2