The Prayer of Fear, Pursuit, Encounter, and Competition
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 63
وَ سَأَلَهُ سَمَاعَةُ بْنُ مِهْرَانَ: عَنْ صَلاَةِ اَلْقِتَالِ فَقَالَ «إِذَا اِلْتَقَوْا فَاقْتَتَلُوا فَإِنَّمَا اَلصَّلاَةُ حِينَئِذٍ تَكْبِيرٌ وَ إِذَا كَانُوا وُقُوفاً لاَ يَقْدِرُونَ عَلَى اَلْجَمَاعَةِ فَالصَّلاَةُ إِيمَاءٌ.
Sama’a ibn Mehran asked him (Abu Abdullah عليه السلام) about the prayer during battle. Imam عليه السلام said: "When they meet and engage in fighting, the prayer at that time is performed with takbir (proclamation of 'Allahu Akbar'). If they are standing and unable to form a congregation, then the prayer is performed with gestures."
- Ṣaḥīḥ al-Faqīh, p. 68