What Can and Cannot Be Used for Prostration
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 41
وَ رَوَى زُرَارَةُ عَنْ أَحَدِهِمَا عَلَيْهِمَا اَلسَّلاَمُ أَنَّهُ قَالَ: قُلْتُ لَهُ اَلرَّجُلُ يَسْجُدُ وَ عَلَيْهِ قَلَنْسُوَةٌ أَوْ عِمَامَةٌ فَقَالَ «إِذَا مَسَّ شَيْءٌ مِنْ جَبْهَتِهِ اَلْأَرْضَ فِيمَا بَيْنَ حَاجِبَيْهِ وَ قُصَاصِ شَعْرِهِ فَقَدْ أَجْزَأَ عَنْهُ.
Zurarah narrated from one of the Imams عليه السلام that He said: "I asked him about a man prostrating while wearing a cap or a turban. Imam عليه السلام said: 'If any part of his forehead touches the ground, between his eyebrows and his hairline, then it suffices.'"
- Ṣaḥīḥ al-Faqīh, p. 41