سنن أبي داود

Narrated Abdullah ibn Umar:

The Messenger of Allah (ﷺ) gave licence to the Mothers of the believers (i.e. the wives of the Prophet) to hang down their lower garment a span. Then they asked him to increase it, and he increased one span for them. They would send (the garment) to us and we would measure it one forearm's length for them.