Sahih al-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 said, 'If the number of the enemy is greater than the Muslims, they can pray while standing or riding (individually).' "
حَدَّثَنَا سَعِيدُ بْنُ يَحْيَى بْنِ سَعِيدٍ الْقُرَشِيُّ، قَالَ حَدَّثَنِي أَبِي قَالَ، حَدَّثَنَا ابْنُ جُرَيْجٍ، عَنْ مُوسَى بْنِ عُقْبَةَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، نَحْوًا مِنْ قَوْلِ مُجَاهِدٍ إِذَا اخْتَلَطُوا قِيَامًا. وَزَادَ ابْنُ عُمَرَ عَنِ النَّبِيِّ صلى الله عليه وسلم " وَإِنْ كَانُوا أَكْثَرَ مِنْ ذَلِكَ فَلْيُصَلُّوا قِيَامًا وَرُكْبَانًا " .
also referenced under: Book 12, Hadith 2 (In-Book) & Volume 2, Book 14, Hadith 65 (USC-MSA)