Ibn Rajab al-Hanbali commented on the virtues of fasting in the heat in ‘Lata’if al-Ma’arif’ (Dal al Kutub al Illmiya, 1st edition, Beirut Lebanon, 1989, pages 365-367)

…And from the acts of worship whose reward is multiplied during the hot summers is fasting, and this is because of the thirst that one experiences in the mid-day heat.

And then he furnishes the following traditions from the companions of the Nabi sala Allahu alayhe was salam:

1. 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.
2. It was related that Abu Bakr would fast in the summer and not fast in the winter
3. Umar al-Khattab advised his son ‘Abdullah ibn Umar on his deathbed: “Try to obtain the characteristics of faith,” and the first one he mentioned was fasting in the intense summer heat.
4. And al-Qasim bin Muhammad (The grandson of Abu Bakr radi Allahu anhu) 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 buys merchandise,” 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.  Meaning in winter time it is easy
5. 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.”
6. And Ibn ‘Umar used to fast extra days until he would almost faint, and this wouldn’t cause him to break his fast.
7. 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 (saw) would pour water over his head while fasting.
8. 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.”