Abu Musa al-Ash’ari (rah) used to fast in hot weather so it as said to him, “You are an old man.”
He said, “I’m preparing it for a long day [i.e. Day of Judgement].
(Read on pg 44, Ibn al-Jawzi, Awakening From The Sleep of Heedlessness. Daar as-Sunnah Publishers. Birmingham: 2012.)