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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.
Source: sunnah.com
Other translations
Arabic
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
sunnah.com