Abu Musa al-Ash’ari was on a boat, and he heard someone calling out: “O passengers, stand up!” And he said this three times. So, Abu Musa told him: “How can we stand up? Don’t you see where we are? How can we stand up?” So, the caller said: “Let me tell you of a rule that Allah made upon Himself: whoever makes himself thirsty for Allah’s sake on a hot day has the right upon Allah to have his thirst quenched on the Day of Resurrection.” So, Abu Musa would search out the days that were so hot that one would feel he was being cooked, and he would fast those days.
Ka’b said that Allah Said to Musa: “I made it incumbent upon Myself that whoever is thirsty for My sake will have his thirst quenched on the Day of Resurrection,” and others said that it’s written in the Torah: “Glad tidings for whoever makes himself hungry in anticipation of the Great Day where he will have his hunger satisfied, and glad tidings for whoever makes himself thirsty in anticipation of the Great Day where he will have his thirst quenched.”