Genesis 21:12

And God said unto Abraham
Either by an articulate voice, or by an impulse on his mind, suggesting to him what he should do, being no doubt in great perplexity how to conduct between his wife and his son, but God determines the case for him, and makes him easy: let it not be grievous in thy sight because of the lad, and because of
the bondwoman:
that is, let not the motion displease thee, which Sarah has made, to turn out the bondwoman and her son; let not thine affection to the one and to the other hinder compliance with it; do not look upon it as an ill thing, or as an hard thing; it is but what is right and proper to be done, and leave the bondwoman and her son to me; I will take care of them, be under no concern for them and their welfare: in all that Sarah hath said unto thee, hearken to her voice;
the Targum of Jonathan adds, for she is a prophetess: and indeed in this affair she spoke under a spirit of prophecy, according to the will of God; at least what she said became a divine oracle, and is called the Scripture, ( Galatians 4:30 ) ; for the word "all" here must be restrained to what she had said concerning Hagar and Ishmael, and their ejection, and not to be extended to everything she had said, or should say to Abraham, to which he was always to be attentive: whereas on the other hand, it became her, as a wife, to hearken and be obedient to the voice of her husband: but in this particular Abraham is bid to listen to her, and do accordingly, for the following reason, for in Isaac shall thy seed be called;
he, and those that descended from him, should be called and reckoned the seed of Abraham more especially; and Abraham's seed in his life should inherit the land of Canaan, given to him and his seed for an inheritance: and this is a good reason why the bondwoman and her son should be cast out, that they or their offspring might not inherit the land with Isaac, or his descendants; and particularly from Abraham in his line, and not in the line of Ishmael, should the Messiah spring, that seed in whom all the nations of the earth should be blessed; and therefore a separation was necessary, that this might abundantly appear.