(2) باب صَلاَةِ الْخَوْفِ رِجَالاً وَرُكْبَانًا رَاجِلٌ قَائِمٌ
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: Sahih al-Bukhari 943
In-book reference: Book 12, Hadith 2
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 14, Hadith 65