كتاب الصلاة
2  
The Book of Prayer
Bulugh al-Maram 368
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."
وَلِلْخَمْسَةِ ‏- وَصَحَّحَهُ اِبْنِ حِبَّانَ ‏- : { صَلَاةُ اَللَّيْلِ وَالنَّهَارِ مَثْنَى مَثْنَى" } وَقَالَ النَّسَائِيُّ : "هَذَا خَطَأٌ" .‏ 1
Reference : Bulugh al-Maram 368
In-book reference : Book 2, Hadith 368
English translation : Book 2, Hadith 363
Hadith Text
Reference