From Sahih Bukhari (Hadith 1946):
Narrated Jabir bin `Abdullah:Allah's Messenger (ﷺ) was on a journey and saw a crowd of people, and a man was being shaded (by them). He asked, "What is the matter?" They said, "He (the man) is fasting." The Prophet (ﷺ) said, "It is not righteousness that you fast on a journey
Reference: Bukhari 1946