Sunan an-Nasa'i, Volume 6, Book 48, Hadith 5341 (DarusSalam) ⓘ
It was narrated that Umm Salamah said: "The Messenger of Allah [SAW] was asked how much a woman should let her hem drag. He said: 'A hand span.' She said: 'But then it will uncover her (feet).' He said: 'A forearm's length, and no more than that.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا النَّضْرُ، قَالَ حَدَّثَنَا الْمُعْتَمِرُ، - وَهُوَ ابْنُ سُلَيْمَانَ - قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنْ سُلَيْمَانَ بْنِ يَسَارٍ، عَنْ أُمِّ سَلَمَةَ، قَالَتْ سُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم كَمْ تَجُرُّ الْمَرْأَةُ مِنْ ذَيْلِهَا قَالَ " شِبْرًا " . قَالَتْ إِذًا يَنْكَشِفَ عَنْهَا . قَالَ " ذِرَاعٌ لاَ تَزِيدُ عَلَيْهَا " .
also referenced under: Hadith 5339 (Reference) & Book 48, Hadith 300 (In-Book)