Whenever he was asked about the witr prayer, he would say, "As for me, if I were to perform witr before sleeping and then wished to pray during the night, I would pair my previous witr with a single rakah, then pray two by two. When I finished my prayer, I would conclude with a single witr rakah. The Messenger of Allah ﷺ instructed that the last prayer of the night should be witr."
حدثنا يعقوب حدثنا أبي عن ابن إسحاق حدثني نافع
أنه كان إذا سئل عن الوتر قال أما أنا فلو أوترت قبل أن أنام ثم أردت أن أصلي بالليل شفعت بواحدة ما مضى من وتري ثم صليت مثنى مثنى فإذا قضيت صلاتي أوترت بواحدة إن رسول الله ﷺ أمر أن يجعل آخر صلاة الليل الوتر
Its isnad is sahih. It appears in Majma al-Zawaid 2:246, where he said, "Ahmad narrated it, and it contains Ibn Ishaq, who is a mudallis, but he is trustworthy, and the rest of its narrators are the narrators of the Sahih." This is an imprecise and unsound assessment, for Ibn Ishaq here explicitly states that he heard it directly from Nafi, which removes any suspicion of tadlis, if such a suspicion ever had any basis. I do not know whether Hafiz Haythami simply overlooked this when reviewing the isnad.In the wording of the hadith as printed in the Zawaid, Ibn Umar's statement at the beginning, "As for me," has dropped out, though it is preserved in the sources here, and is also preserved in al-Muntaqa 1217, where it is transmitted from the Musnad. See the previous hadith.