Ibn Rajab commented on the virtues of fasting in the heat in 'Lata'if al-Ma'arif' (p. 272-273):
"...And from the acts of worship whose reward is multiplied during the heat is fasting, and this is because of the thirst that one experiences in the mid-day heat.
This is why Mu'adh bin Jabal expressed regret on his deathbed that he would no longer experience this mid-day thirst, as did other early Muslims.
And it was related that Abu Bakr would fast in the summer and not fast in the winter, and 'Umar advised his son 'Abdullah on his deathbed: "Try to obtain the characteristics of faith," and the first one he mentioned was fasting in the intense summer heat.
And al-Qasim bin Muhammad said that 'A'ishah would fast in the intense heat, and he was asked: "What drove her to do this?" He replied: "She would take advantage of the days before death." And some of the righteous women would choose the hottest days and fast them, saying: "If the price is low, everyone will buy," meaning that she wanted to do those actions that only a few were capable of due to how hard it was to do them, and this is indicative of the high aspirations these women had.
And 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."
al-Hasan said: "A maiden of Paradise will speak to the wali of Allah while he is laying with her on the shore of a river of honey in Paradise while she hands him a glass of the sweetest drink, and she will ask him: "Do you know what day Allah married me to you? He Saw you on a long summer day while you were thirsty in the mid-day heat, and He called the Angels and Said: "Look at My slave. He has left his wife and pleasure and food and drink for Me out of his desire for what I have for him. Bear witness that I have Forgiven him," and He Forgave you on that day and married you to me.""
And when 'Amir bin 'Abd Qays went from Basrah to Sham, Mu'awiyah would ask him to tell him what he needed. He refused to ask of him, and eventually said: "All I need is for you to return the heat of Basrah to me to make the fasting a bit harder, as it is too easy in your lands."
And al-Hajjaj was on a journey between Makkah and Madinah. He pulled out his dinner and invited a bedouin to eat with him, and the bedouin said: "I have been invited by One who is better than you and I have accepted the invitation." He asked: "And who is this?" The man replied: "Allah invited me to fast, and I fasted." al-Hajjaj asked: "On this very hot day?" The man replied: "Yes. I am fasting it in anticipation of a much hotter day." al-Hajjaj said: "So, eat today and fast tomorrow." The man replied: "Only if you can guarantee that I will live until tomorrow." al-Hajjaj said: "This isn't in my hands." The man said: "How can you ask me to do something now when there is something of the future that isn't in your hands?"
And Ibn 'Umar went on a trip once with some companions, and they saw a sheep-herder who they invited to eat with them. He said: "I am fasting," and Ibn 'Umar said: "You are fasting in heat like this, and while you are between all these plants and sheep?" The herder replied: "I'm taking advantage of my remaining days." Ibn 'Umar was impressed by this reply and said: "Can you sell one of your sheep to us? We'll feed you from its meat when you break your fast, and we'll also pay you for it." The herder said: "It doesn't belong to me. It belongs to my master." Ibn 'Umar said: "What would your master say if you told him that it was eaten by a wolf?" The herder raised his finger to the sky and said: "What about Allah?" Ibn 'Umar kept repeating this phrase that the herder was saying, and when he got to the city, he went to the herder's owner and bought him and his sheep from him. He then freed the herder and gave him his sheep as a gift.
And Ruh bin Zinba' was traveling between Makkah and Madinah on a very hot day. A herder living on a mountain approached him, and he said to him: "O herder, come eat with me." The herder said: "I'm fasting." Ruh said: "You're fasting in this heat?" The herder replied: "Should I let my days pass by in vain?" ٍSo, Ruh said: "You have used your days wisely, O herder, while Ruh bin Zinba' has wasted his."
And Ibn 'Umar used to fast extra days until he would almost faint, and this wouldn't cause him to break his fast. And al-Imam Ahmad would fast until he was about to pass out, and would wipe water over his face. He was asked about fasting very hot days, and he replied: "There is nothing wrong with wetting a towel to squeeze the water on himself to cool down with." And the Prophet (صلى الله عليه و سلم) would pour water over his head while fasting.
ِAnd Abu ad-Darda' would say: "Fast the very hot days in anticipation of the Day of Resurrection, and pray two rak'at in the darkness of night in anticipation of the darkness of the grave." And it's reported in the two 'Sahih's that he said: "You have seen us with the Messenger of Allah (صلى الله عليه و سلم) on some of his journeys on very hot days, and a man would hold his hand against his head due to the intensity of the heat, and none of them would be fasting except the Messenger of Allah and 'Abdullah bin Rawahah." And the narration of Muslim states that Abu ad-Darda' said: "This was during the month of Ramadan."
When those who fast for Allah in the heat are patient despite their intense thirst, He set aside a specific gate of the gates of Paradise for them. This is the gate called Rayyan, and whoever enters through it will drink, and whoever drinks after entering it will never be thirsty again. When they enter through it, it will be locked for those after them, and none will enter through it except them..."