The Etiquette of Women in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 55
قَالَ وَ سَأَلْتُهُ: عَنِ اَلْأَمَةِ إِذَا وَلَدَتْ عَلَيْهَا اَلْخِمَارُ قَالَ «لَوْ كَانَ عَلَيْهَا لَكَانَ عَلَيْهَا إِذَا هِيَ حَاضَتْ وَ لَيْسَ عَلَيْهَا اَلتَّقَنُّعُ فِي اَلصَّلاَةِ.
I asked him about a slave woman (amā) who gave birth, and whether the headcover (khimar) would be required for her. Imam عليه السلام said: "If it were required for her, She would have to wear it when She menstruates, but there is no requirement for her to wear a headcover in prayer."
- Ṣaḥīḥ al-Faqīh, p. 51