Jamieson, Fausset and Brown on Genesis 21:10

10. Wherefore she said unto Abraham, Cast out this bondwoman--Nothing but the expulsion of both could now preserve harmony in the household. Abraham's perplexity was relieved by an announcement of the divine will, which in everything, however painful to flesh and blood, all who fear God and are walking in His ways will, like him, promptly obey. This story, as the apostle tells us, in "an allegory" [Ga 4:24], and the "persecution" by the son of the Egyptian was the commencement of the four hundred years' affliction of Abraham's seed by the Egyptians.