Musnad Abu Ya'la al-Mawsili 209:63

Narrated to us Sufyan ibn Waki', narrated to us Abd al-Wahhab al-Thaqafi, meaning from Khalid, from Abu Qilabah, from Zaynab bint Umm Salamah, from Umm Salamah, who said: My bedding was opposite the prayer place of the Messenger of Allah (ﷺ), and he would pray while I was opposite him.