وَ كَتَبَ مُحَمَّدُ بْنُ اَلْحَسَنِ اَلصَّفَّارُ رَضِيَ اَللَّهُ عَنْهُ إِلَى أَبِي مُحَمَّدٍ اَلْحَسَنِ بْنِ عَلِيٍّ عَلَيْهِ اَلسَّلاَمُ: فِي رَجُلٍ أَرَادَ أَنْ يَشْهَدَ عَلَى اِمْرَأَةٍ لَيْسَ لَهَا بِمَحْرَمٍ هَلْ يَجُوزُ لَهُ أَنْ يَشْهَدَ عَلَيْهَا مِنْ وَرَاءِ اَلسِّتْرِ وَ يَسْمَعَ كَلاَمَهَا إِذَا شَهِدَ عَدْلاَنِ أَنَّهَا فُلاَنَةُ بِنْتُ فُلاَنٍ اَلَّتِي تُشْهِدُكَ وَ هَذَا كَلاَمُهَا أَوْ لاَ تَجُوزُ اَلشَّهَادَةُ عَلَيْهَا حَتَّى تَبْرُزَ وَ تُثْبِتَهَا بِعَيْنِهَا فَوَقَّعَ عَلَيْهِ اَلسَّلاَمُ "تَتَنَقَّبُ وَ تَظْهَرُ لِلشُّهُودِ إِنْ شَاءَ اَللَّهُ".
It is narrated that Muhammad ibn al-Hasan al-Saffar (may Allah (swt) be pleased with him) wrote to Abu Muhammad al-Hasan ibn Ali (a.s) regarding a man who wanted to testify about a woman who was not his mahram (unmarriageable relative). He asked: "Is it permissible for him to testify about her from behind a curtain while hearing her speech, provided that two just witnesses testify that she is indeed so-and-so, the daughter of so-and-so, who is calling you to bear witness and this is her voice? Or is it not permissible to testify about her until she uncovers her face and is identified by her appearance?" The Imam (a.s) replied: "She should veil her face partially (wear a niqab) and appear before the witnesses, if Allah (swt) wills."