John Wesley on Genesis 21:9

Sarah saw the son of the Egyptian mocking - Mocking Isaac no doubt, for it is sad, with reference to this, Gal 4:29, that he that was born after the flesh, persecuted him that was born after the spirit. Ishmael is here called the son of the Egyptian, because (as some think) the four hundred years affliction of the seed of Abraham by the Egyptians began now, and was to be dated from hence.