Or HaChaim on Exodus 15:26/אור החיים על שמות ט״ו:כ״ו

1 אם שמוע תשמעו לקול ה׳ אלוקיך, "if you will carefully listen to the voice of the Lord your G'd, etc." This means that if the Israelites would hearken to the statutes and social legislation Moses introduced in the name of G'd immediately after this experience and they would observe them, they would also merit to listen to His other commandments and hear these commandments directly from G'd's mouth. [the reference is to the impending revelation at Mount Sinai. Ed.] In Deut. 4,33 Moses reminded the Israelites of their experience when he asked rhetorically: "Has any nation ever heard the sound of G'd's voice out of the fire and remained alive as you have done?" According to Sanhedrin 56 G'd gave the Israelites some of the Sabbath legislation as well as the laws of דינים, jurisprudence, while they were at Marah. The words שם ניסהו, "there He tested them," must be understood as a trial preceding the revelation at Mount Sinai. If the Israelites would prove receptive to the laws revealed at Marah there was reason to believe they would accept the entire Torah at Sinai when the time came.

2Actually, the Torah alludes to four distinct commandments here, ללמוד וללמד, לשמור ולעשות, to study the law, to teach it, to observe the prohibitions, and to carry out the aspects demanding action. The words: אם שמוע תשמע allude to the duty to study these laws. The repetition implies the demand that one should commit what one has learned to memory. It also suggests that one should grow fond of studying G'd's laws. One should not feel one has already done enough studying. The words: והישר בעיניו תעשה, "you will do what is right in His eyes," refer to the teaching of G'd's laws for free, similar to what we have been told in Nedarim 37 that just as Moses did not charge us for teaching the Torah, so we should not charge others for teaching them the Torah. The word ישר refers to G'd who in His goodness gave the Torah to His creatures expecting His creatures in turn to hand it on for free. The words: והאזנת למצותיו, imply carrying out positive commandments, whereas the words: ושמרת כל חוקיו imply not violating negative commandments.

3אשר שמתי במצרים, "which I put upon the Egyptians, etc." The reason G'd mentioned that He had brought diseases upon the Egyptians, something we are all aware of, is to prevent us from making an error. If G'd had not referred to the past we could have thought that He only promised not to inflict these diseases upon us, whereas if we contracted them without His active intervention we would not be protected against them. G'd reminds us that just as He made sure we were not infected by the diseases with which He struck the Egyptians so we would remain totally immune to any of the germs which caused these diseases in the Egyptians.

4כי אני ה׳ רפאך, "For I the Lord am your Healer." The Torah here refers to diseases which are not due to Divine intervention such as the common cold contracted due to one's carelessness, etc. (compare Ketuvot 30). G'd undertakes to protect us even against such diseases which are not normally controlled by Him if we live according to His precepts.