Sunan Abi Dawud 4119
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.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ سُفْيَانَ، أَخْبَرَنِي زَيْدٌ الْعَمِّيُّ، عَنْ أَبِي الصِّدِّيقِ النَّاجِيِّ، عَنِ ابْنِ عُمَرَ، قَالَ رَخَّصَ رَسُولُ اللَّهِ صلى الله عليه وسلم لأُمَّهَاتِ الْمُؤْمِنِينَ فِي الذَّيْلِ شِبْرًا ثُمَّ اسْتَزَدْنَهُ فَزَادَهُنَّ شِبْرًا فَكُنَّ يُرْسِلْنَ إِلَيْنَا فَنَذْرَعُ لَهُنَّ ذِرَاعًا ‏.‏