Genesis 21:15

And it was when they came to the entrance of the desert, they remembered to wander after strange worship; and Ishmael was seized with a burning thirst, and drank of the water till all the water was consumed from the cruse. And he was dried up, and withered in his flesh; and she carried him, and was exhausted, and she cried unto the Fear of his father, and He answered her not; and she laid the youth down at once under one of the trees.

AND SHE CAST THE CHILD. Thirst overtook him and he was unable to walk, and so his mother laid him under the tree, cast away and abandoned. It may be that the word vatashleich (and she cast) is similar in sense to the verses: And He cast them into another land; Cast me not away from Thy presence, meaning “sending away.”
Rabbi Abraham ibn Ezra said: “And she cast for she had taken him onto her lap when he was weakened by thirst, [and seeing that he was expiring from thirst, she cast him from her].”
Our Rabbis have said that he was sick at the time he sent him away, and therefore he put the child on her shoulder. This is the sense of the word vatashleich (and she cast) him: [until that point she had carried him].
All this occurred to Abraham because he had been commanded to do whatever Sarah said, and she commanded that he send him away immediately, and it was at her command that he did not give them silver and gold, servants, and camels to bear them.

ויכלו, Hagar’s problems became ever more acute. The reason why the Torah writes all this detail, is to teach that if man is confident that G’d will help him overcome his difficulties in the end, then his troubles do not multiply in order to teach him to appeal to G’d for help. The detail here has also been written in order to show how G’d deals with people who love His name. He appeared to Hagar on two separate occasions rescuing her from acute distress, performing miracles on behalf of her son.

ותשלך את הילד, whom she had been carrying, seeing that she had become too tired to carry on. When she saw that she had been unable to locate any water, she threw him from her lap under a shrub. According to Onkelos שיחים are small trees. [it was doubly frustrating to know that there must have been a source of water nearby, how else could these trees exist? Ed.]

ויכלו המים, “the water was used up;” Rashi adds that it is the custom of sick people to drink a lot of water, so that the water was not used up as it had been insufficient for the journey to start with. This interpretation is somewhat puzzling, as we have been told in the Talmud, tractate Baba metzia, folio 87, that prior to the report in the Torah about Yaakov falling sick shortly before his death, sickness as we know it did not exist. We would have to conclude therefore that what the Talmud meant that until Yaakov fell sick, for no visible cause, no one fell sick without visible cause. Yishmael’s “sickness” was not a sickness in the accepted meaning of the word but was the result of Sarah having used the “evil eye” to cause this.