When the Messenger of Allah ﷺ mentioned lower garment, Umm Salamah, wife of the Messenger of Allah ﷺ, asked him: And a woman, Messenger of Allah? He replied: She may hang down a span. Umm Salamah said: Still it (foot) will be uncovered. He said: Then a forearm's length, nor exceeding it.  

أَنَّ أُمَّ سَلَمَةَ زَوْجَ النَّبِيِّ ﷺ قَالَتْ لِرَسُولِ اللَّهِ ﷺ حِينَ ذَكَرَ الإِزَارَ فَالْمَرْأَةُ يَا رَسُولَ اللَّهِ قَالَ تُرْخِي شِبْرًا قَالَتْ أُمُّ سَلَمَةَ إِذًا يَنْكَشِفُ عَنْهَا قَالَ فَذِرَاعًا لاَ تَزِيدُ عَلَيْهِ  

Ibn Ishaq and Ayyub b. Musa transmitted it from Nafi' from Safiyyah.  

عَنِ النَّبِيِّ ﷺ بِهَذَا الْحَدِيثِ  

The Messenger of Allah ﷺ gave licence to the Mothers of the believers (i.e. the wives of the Prophet) to hang down their lower garment a span. Then they asked him to increase it, and he increased one span for them. They would send (the garment) to us and we would measure it one forearm's length for them.  

رَخَّصَ رَسُولُ اللَّهِ ﷺ لأُمَّهَاتِ الْمُؤْمِنِينَ فِي الذَّيْلِ شِبْرًا ثُمَّ اسْتَزَدْنَهُ فَزَادَهُنَّ شِبْرًا فَكُنَّ يُرْسِلْنَ إِلَيْنَا فَنَذْرَعُ لَهُنَّ ذِرَاعًا