Sunnah.com reference : Book 2, Hadith 260English translation : Book 2, Hadith 352Arabic reference : Book 2, Hadith 357
Narrated Umm Habibah (RA) the Mother of the Believers:
She heard Allah's Messenger (ﷺ) say, "Whoever prays twelve (voluntary) Rak'at in a day and a night, a house will be built for him in Paradise due to them (the Rak'at)."

[Reported by Muslim].

In another narration, it is mentioned: "Voluntarily".
وَعَنْ أُمِّ حَبِيبَةَ أُمِّ اَلْمُؤْمِنِينَ ‏-رَضِيَ اَللَّهُ عَنْهَا‏- قَالَتْ : سَمِعْتَ اَلنَّبِيَّ ‏- صلى الله عليه وسلم ‏-يَقُولُ : { مَنْ صَلَّى اِثْنَتَا عَشْرَةَ رَكْعَةً فِي يَوْمٍ وَلَيْلَةٍ بُنِيَ لَهُ بِهِنَّ بَيْتٌ فِي اَلْجَنَّةِ }  رَوَاهُ مُسْلِمٌ .‏ وَفِي رِوَايَةٍ 
" تَطَوُّعًا" .‏  1‏‏1 ‏- صحيح .‏ رواه مسلم (728 )‏ .‏