A man came to the Prophet ﷺ and asked him about the night prayer. He said, "The night prayer is in pairs of two. You give the salam after every two units. When you fear the approach of dawn, pray a single unit, which will make all that you prayed before it odd-numbered."
حدثنا محمد بن عبد الله بن الزبير يعني أبا أحمد الزبيري قال حدثنا عبد العزيز يعني ابن أبي رواد عن نافع
جاء رجل إلى النبي ﷺ فسأله عن صلاة الليل فقال صلاة الليل مثنى مثنى تسلم في كل ركعتين فإذا خفت الصبح فصل ركعة توتر لك ما قبلها
Its isnad is sahih. Muhammad bin Abdullah bin Zubayr Asadi, Abu Ahmad Zubayri, is trustworthy and one of the shaykhs of Ahmad. His narrations have come up repeatedly before, and an error of his was pointed out earlier in the isnad of 517. Ibn Main and others deemed him trustworthy, and Ibn Numayr said, "Trustworthy, sound in his written material." Bundar said, "I have not seen anyone with a stronger memory than him." Ahmad said, "He made many errors in the hadith of Sufyan," even though Zubayri himself said, "I would not mind if Sufyan's book were stolen from me, for I have memorized all of it." Bukhari mentions him in Kabir, 1/1/133–134.This hadith is a repetition of 5085, and see also 5096.