The Book of the Commencement of the Prayer
11/69. Chapter:
The virtue of reciting "Say: He is Allah, (the) One"
Sunan an-Nasai
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ عُبَيْدِ بْنِ حُنَيْنٍ، مَوْلَى آلِ زَيْدِ بْنِ الْخَطَّابِ قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ، يَقُولُ أَقْبَلْتُ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَسَمِعَ رَجُلاً يَقْرَأُ { قُلْ هُوَ اللَّهُ أَحَدٌ * اللَّهُ الصَّمَدُ * لَمْ يَلِدْ وَلَمْ يُولَدْ * وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ } فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " وَجَبَتْ " . فَسَأَلْتُهُ مَاذَا يَا رَسُولَ اللَّهِ قَالَ " الْجَنَّةُ " .
It was narrated that Ubaid bin Hunain, the freed slave of the family of Zaib bin Al-Khattab, said: "I heard Abu Hurairah say: 'I came back (from a journey) with the Messenger of Allah (ﷺ) and he heard a man reciting 'Say: He is Allah, (the) One, Allah-us-Samad (the Self-Sufficient Master). He begets not, nor was He begotten. And there is none equal or comparable unto Him.' The Messenger of Allah (ﷺ) said: 'It is guaranteed.' We asked him: 'What, O Messenger of Allah?' He said: 'Paradise.'"
Reference : Sunan an-Nasa'i 994In-book reference : Book 11, Hadith 119English translation : Vol. 2, Book 11, Hadith 995