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]‏