Commentaire sur La Genèse 21:15

ויכלו המים AND THE WATER WAS SPENT, because it is the nature of sick people to drink much (Genesis Rabbah 53:13).

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;³³³Deuteronomy 29:27. Cast me not away from Thy presence,³³⁴Psalms 51:13. 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 said³³⁵Bereshith Rabbah 53:17. 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.

Under one of the bushes: To protect against the burning sun and it adding to [his] thirst.