Bulugh al-Maram
The Book of Prayer — Hadith #435
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 ) .
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).”
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In another narration, it is mentioned: “Voluntarily”.