The Prayer of Fear, Pursuit, Encounter, and Competition
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 63
وَ سَأَلَ سَمَاعَةُ بْنُ مِهْرَانَ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَنِ اَلرَّجُلِ يَأْخُذُهُ اَلْمُشْرِكُونَ فَتَحْضُرُهُ اَلصَّلاَةُ فَيَخَافُ مِنْهُمْ أَنْ يَمْنَعُوهُ قَالَ «يُومِئُ إِيمَاءً.
Sama'a ibn Mehran asked Imam Jafar ibn Muhammad Al-Sadiq عليه السلام about a man who is captured by polytheists, and the time for prayer arrives, but he fears that they will prevent him from praying. Imam عليه السلام said: "He should pray by gesturing."
- Ṣaḥīḥ al-Faqīh, p. 67