بُلُوغُ الْمَرَامِ
Bulugh al-Maram 357
The Book of Prayer
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 . [1]
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".
Sunnah.com reference: Book 2, Hadith 260