He had been touched on the face by the Messenger of Allah ﷺ. He saw Saad bin Abu Waqqas perform witr with a single rakah, not adding to it, until he would rise in the depth of the night.
حدثنا عبد الله بن الحارث قال قرأه علي يونس عن ابن شهاب قال أخبرني
وكان رسول الله ﷺ مسح وجهه أنه رأى سعد بن أبي وقاص يوتر بركعة واحدة لا يزيد عليها حتى يقوم من جوف الليل
Its isnad is sahih. Abdullah bin Harith is Ibn Abdul-Malik Makhzumi, previously mentioned, and he is trustworthy, his hadith is found in Muslim. Bukhari narrated the hadith in 11/151, number 6356, Fath al-Bari, in the Book of Supplications, on supplicating for children with blessing.