Musnad Ahmad 27:236

Yahya narrated to us, from Sufyan, Zayd al-Ammi narrated to me, from Abu al-Siddiq, from Ibn Umar, who said: "The Messenger of Allah (may Allah bless him and grant him peace) granted a concession to the Mothers of the Believers regarding the train [of their garments] of one span. They asked him for more, so he granted them another span, making it a cubit. They used to send [the garments] to us, and we would measure a cubit for them."