וַיֵּצֵא֙ אֶל־מוֹצָ֣א הַמַּ֔יִם וַיַּשְׁלֶךְ־שָׁ֖ם מֶ֑לַח וַיֹּ֜אמֶר כֹּֽה־אָמַ֣ר יְהוָ֗ה רִפִּ֙אתִי֙ לַמַּ֣יִם הָאֵ֔לֶּה לֹֽא־יִהְיֶ֥ה מִשָּׁ֛ם ע֖וֹד מָ֥וֶת וּמְשַׁכָּֽלֶת׃
Rashi on II Kings
And threw salt there. Now is not salt a thing that spoils water? This was a miracle within a miracle.20This miracle was similar to the one performed by Moshe in Marah. After traveling through the wilderness for three days without water, the Bnei Yisroel finally came to a spring only to find the water undrinkably bitter. Moshe threw a tree whose wood was bitter into the water and the water became sweet and drinkable. See Shemos 15:22-26.—Radak