بُلُوغُ الْمَرَامِ
Bulugh al-Maram 396
The Book of Prayer
وَعَنْ أَنَسٍ قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ مَنْ صَلَّى اَلضُّحَى ثِنْتَيْ عَشْرَةَ رَكْعَةً بَنَى اَللَّهُ لَهُ قَصْرًا فِي اَلْجَنَّةِ } رَوَاهُ اَلتِّرْمِذِيُّ وَاسْتَغْرَبَهُ . [1] .
Narrated Anas (RA): Allah's Messenger (ﷺ) said: "Whoever prays twelve Rak'at of Duha, Allah will build a castle for him in Paradise." [Reported by at-Tirmidhi who graded it Gharib (reported by a single narrator)].
Sunnah.com reference: Book 2, Hadith 300