Musannaf Ibn Abi Shayba 16:661

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Sufyan, from Zayd al-Ammi, from Abu al-Siddiq, from Ibn Umar, that the wives of the Prophet (may Allah bless him and grant him peace) were granted a concession for the train [of their garments] of one cubit. So they would come to us, and we would measure for them a cubit with a reed.