Surah Al-Baqarah 2:239

فَإِنْ خِفْتُمْ فَرِجَالًا أَوْ رُكْبَانًا ۖ فَإِذَآ أَمِنتُمْ فَٱذْكُرُوا۟ ٱللَّهَ كَمَا عَلَّمَكُم مَّا لَمْ تَكُونُوا۟ تَعْلَمُونَ

And if you fear [an enemy, then pray] on foot or riding. But when you are secure, then remember Allāh [in prayer], as He has taught you that which you did not [previously] know.

Translation: Saheeh International

Tafsir

And if you are in fear of an enemy or a torrent or a predatory animal then standing rijāl plural of rājil praying while walking or mounted rukbān plural of rākib. In other words in whichever way you can facing the direction of the qibla or otherwise making the gestures of genuflexion and prostration; …

Excerpt from Tafsir al-Jalalayn, commentary on 2:239, via Quran.com. The reader has the full commentary and 9 more tafsirs.