Sahih Muslim 1695
The Book of Prayer - Travellers
حَدَّثَنِي أَبُو غَسَّانَ الْمِسْمَعِيُّ، حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ، حَدَّثَنَا دَاوُدُ، عَنِ النُّعْمَانِ بْنِ سَالِمٍ، بِهَذَا الإِسْنَادِ " مَنْ صَلَّى فِي يَوْمٍ ثِنْتَىْ عَشْرَةَ سَجْدَةً تَطَوُّعًا بُنِيَ لَهُ بَيْتٌ فِي الْجَنَّةِ " .
Nu'man b. Salim reported with the same chain of transmitters:He who observed twelve voluntary rak'ahs, a house will be built for him in Paradise
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.