Sunan Ibn Majah
3580
Sahih+3

It was narrated that Umm Salamah said:“The Messenger of Allah (ﷺ) was asked about how long a woman’s hem should hang down. He said: ‘A hand span.’ I said: ‘But then (her legs and feet) will be uncovered.’ He said: ‘Then a forearm’s length, but no more than that.’”

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi