From Sahih Bukhari (Hadith 943):
Narrated Nafi`:Ibn `Umar said something similar to Mujahid's saying: Whenever (Muslims and non-Muslims) stand face to face in battle, the Muslims can pray while standing. Ibn `Umar added, "The Prophet (ﷺ) said, 'If the number of the enemy is greater than the Muslims, they can pray while standing or riding (individually)
Reference: Bukhari 943