Hadithcore
Bulugh al-Maram, 86

Ibn Hibban also reported the above Hadith with the version:

“Let him say in his heart...”

وَأَخْرَجَهُ اِبْنُ حِبَّانَ بِلَفْظِ: { فَلْيَقُلْ فِي نَفْسِهِ } [1]‏ .‏

References2 variants
In-Book Reference
Book 1, Hadith 104
English Reference
Book 1, Hadith 92