Musnad Ahmad 27:1187

Hajjaj narrated to us, Sharik informed us, from Mutarrif, from Zayd al-'Ammi, from Abu al-Siddiq al-Naji, from Ibn 'Umar, that the wives of the Prophet, may Allah bless him and grant him peace, asked him about the train (of a garment). He said: "Make it a span." They said: "A span does not cover the 'awrah (private parts)." He said: "Make it a cubit." So, when one of them wanted to make a garment, she would let down a cubit and make it a train.