Musannaf Ibn Abi Shayba 3:6223

Waki’ narrated to us, from Dawud ibn Qays, from ‘Abdullah ibn Rafi’, from Umm Salamah, that she had a copy of the Mushaf written. When she reached [the verse]: "Guard strictly the prayers, especially the middle prayer," she said: "Write: 'and the afternoon prayer (al-‘asr).'"