Sa'īd b. al-Musayyib reported in mursal form that the Prophet (ﷺ) said, “If anyone recites ten times ‘Say, He is God, One’, a palace will be built for him in paradise because of it; if anyone recites twenty times two palaces will be built for him in paradise because of it; and if anyone recites it thirty times three palaces will be built for him in paradise because of it.” ‘Umar b. al-Khattāb said, “I swear by God, messenger of God, that we shall then produce many palaces for ourselves;” to which he replied, “God's abundant grace is even more comprehensive than that.” Dārimī transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ سَعِيدِ بْنِ الْمُسَيَّبِ مُرْسَلًا عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «مَنْ قَرَأَ (قل هُوَ الله أحد) عشر مَرَّات بني لَهُ بِهَا قَصْرٌ فِي الْجَنَّةِ وَمَنْ قَرَأَ عِشْرِينَ مَرَّةً بُنِي لَهُ بِهَا قَصْرَانِ فِي الْجَنَّةِ وَمَنْ قَرَأَهَا ثَلَاثِينَ مَرَّةً بُنِيَ لَهُ بِهَا ثَلَاثَةُ قُصُورٍ فِي الْجَنَّةِ» . فَقَالَ عُمَرُ بْنُ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ: وَاللَّهِ يَا رَسُولَ اللَّهِ إِذَا لَنُكَثِّرَنَّ قُصُورَنَا. فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «اللَّهُ أَوْسَعُ من ذَلِك» . رَوَاهُ الدَّارمِيّ
sunnah.com