It is narrated from Nafi' that the Mother of the Believers, Sayyidah Hafsah (may Allah be pleased with her), instructed the scribe regarding the copy of the Qur'an (mushaf) that was being written for her: "When you reach the mention of the prayer, leave its place blank, so that I may tell you what I heard the Messenger of Allah (peace and blessings be upon him) reciting." So he did so. She (may Allah be pleased with her) dictated: "Guard the prayers, and the middle prayer, and the Asr prayer too." Nafi' said: I saw the waw (the letter 'and') written hanging.
This report is musnad (connected to the Prophet), except that there is irsal (a gap) on the part of Nafi'; he then confirmed it with what he reported of his own observation. And the report of Zayd bin Aslam through Amr the scribe is mawsul (connected); even though it is mawquf, it is a supporting witness to the soundness of the report of Ubaydullah bin Umar from Nafi'.
Through another chain it is transmitted from Umar bin Rafi', the freed slave of Sayyiduna Umar bin al-Khattab (may Allah be pleased with him): I used to write copies of the Qur'an in the time of the Mothers of the Believers, and Sayyidah Hafsah bint Umar (may Allah be pleased with her) had me write a copy for her. She said to me: "My son, when you reach this verse, 'Guard the prayers, especially the middle prayer' [Surah al-Baqarah: 238], do not write it until you bring it to me, and I will dictate it to you as I memorised it from the Messenger of Allah (peace and blessings be upon him)." He said: When I reached this verse, I took the sheet and the inkpot to her. She said: "Write: 'Guard the prayers, and the middle prayer, which is the Asr prayer, and stand before Allah in devout obedience.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2175