Imam Bukhari narrates: Sa'id bin Yahya bin Sa'id al-Qurashi narrated to us, he said: My father narrated to me, he said: Ibn Juraij narrated to us from Musa bin Uqbah from Nafi' from Ibn Umar, he narrated the like of the saying of Mujahid, that

when they are standing engaged in close combat. And Ibn Umar, may Allāh be pleased with them both, added in the narration from the Prophet ﷺ that if the disbelievers are greater in number than this, then they should pray while walking on foot, or while riding.