The Prophet ﷺ used to perform witr in three units, reciting nine surahs from the Mufassal section. In each unit he would recite three surahs, the last of them being "Say, He is Allah, the One."
حدثنا هناد حدثنا أبو بكر بن عياش عن أبي إسحق عن الحارث
كان النبي ﷺ يوتر بثلاث يقرأ فيهن بتسع سور من المفصل يقرأ في كل ركعة بثلاث سور آخرهن قل هو الله أحد
[Its chain of transmission is very daif] Ahmad narrated it, 1/84, from the hadith of Abu Bakr bin Ayyash with the same wording in a shortened form, and Israil also narrated it from Abu Ishaq with the same wording. Harith Awar is very daif.There are also narrations on this subject from Imran bin Husayn (the note points to the hadith in Nasai, no. 1744), from Ayesha (mentioned earlier at no. 439), from Ibn Abbas (coming later at no. 462), from Abu Ayub (already mentioned in the referencing of no. 458), and from Abdul-Rahman bin Abza, from Ubayy bin Kaab, in Ibn Majah, no. 1182, and Nasai, no. 1700.The hadith of Abdul-Rahman bin Abza from the Prophet ﷺ, found in Nasai, and the report of Hisham bin Hassan from Muhammad bin Sirin, is daif. Hisham narrated with "an" and he is a mudallis, but the wording is sahih on the basis of other evidence.