(151) باب قَوْلِ النَّبِيِّ صلى الله عليه وسلم " كُلُّ صَلاَةٍ لاَ يُتِمُّهَا صَاحِبُهَا تَتِمُّ مِنْ تَطَوُّعِهِ "
The above-mentioned tradition has also been transmitted by Abu Hurairah through a different chain of narrators to the same effect.
Reference: Sunan Abi Dawud 865
In-book reference: Book 2, Hadith 475
English translation: Book 2, Hadith 864