Mishkat al-Masabih 2160
Abū Huraira said:
The Prophet, on hearing a man reciting “Say, He is God, One” remarked, “It is certain.” I asked him what was certain, and he replied that it was paradise. 

Mālik, Tirmidhī and Nasā’i transmitted it.
وَعَنْ أَبِي هُرَيْرَةَ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ سَمِعَ رَجُلًا يَقْرَأُ (قُلْ هُوَ اللَّهُ أَحَدٌ)
فَقَالَ: «وَجَبَتْ» قُلْتُ: وَمَا وَجَبَتْ؟ قَالَ: «الْجنَّة» . رَوَاهُ مَالك وَالتِّرْمِذِيّ وَالنَّسَائِيّ