بُلُوغُ الْمَرَامِ
Bulugh al-Maram 368
The Book of Prayer
وَلِلْخَمْسَةِ - وَصَحَّحَهُ اِبْنِ حِبَّانَ - : { صَلَاةُ اَللَّيْلِ وَالنَّهَارِ مَثْنَى مَثْنَى" } وَقَالَ النَّسَائِيُّ : "هَذَا خَطَأٌ" . [1]
The Khamsah reported it - graded Sahib (authentic) by Ibn Hibban - with the wording: "the (voluntary) day and night prayers are two Rak'at, two Rak'at (in pairs)." an-Nasa'i said, "This is a mistake."
Sunnah.com reference: Book 2, Hadith 272