The Messenger of Allah ﷺ sent a man on a campaign, and he used to recite to his companions when leading them in prayer, and would conclude with 'Say: He is Allah, (the) One.' When they returned, they told the Messenger of Allah ﷺ about that. He said: "Ask them why he did that." So they asked him and he said: "Because it is a description of the Most Merciful, the Mighty and Sublime, and I love to recite it." The Messenger of Allah ﷺ said: "Tell him that Allah (SWT), the Mighty and Sublime, loves him." (Using translation from Nasāʾī 993)  

أَنَّ رَسُولَ اللهِ ﷺ بَعَثَ رَجُلًا عَلَى سَرِيَّةٍ وَكَانَ يَقْرَأُ لِأَصْحَابِهِ فِي صَلَاتِهِمْ فَيَخْتِمُ بِقُلْ هُوَ اللهُ أَحَدٌ فَلَمَّا رَجَعُوا ذَكَرُوا ذَلِكَ لِرَسُولِ اللهِ ﷺ فَقَالَ «سَلُوهُ لِأَيِّ شَيْءٍ صَنَعَ ذَلِكَ؟» فَسَأَلُوهُ فَقَالَ «لِأَنَّهَا صِفَةُ الرَّحْمَنِ فَأَنَا أَحَبُّ أَنْ أَقْرَأَ بِهَا» قَالَ رَسُولُ اللهِ ﷺ «أَخْبِرُوهُ أَنَّ اللهَ يُحِبُّهُ»  

"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.'" (Using translation from Nasāʾī 994)  

سَمِعْتُ أَبَا هُرَيْرَةَ يَقُولُ أَقْبَلْتُ مَعَ رَسُولِ اللهِ ﷺ فَسَمِعَ رَجُلًا يَقْرَأُ {قُلْ هُوَ اللهُ أَحَدٌ اللهُ الصَّمَدُ لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ} [الإخلاص 2] فَقَالَ رَسُولُ اللهِ ﷺ «وَجَبَتْ» فَسَأَلْتُ مَاذَا يَا رَسُولَ اللهِ؟ قَالَ «الْجَنَّةُ»  

A man heard another man reciting "Say, He is Allah, One" He was repeating it. When the next morning came, he went to the Messenger of Allah ﷺ and mentioned to him. The man tool it (this surah) as a small one. The Prophet ﷺ said: By Him in Whose Hand is my life, it is equivalent to a third of the Qur'an. (Using translation from Abū Dāʾūd 1461)  

سَمِعَ رَجُلًا يَقْرَأُ قُلْ هُوَ اللهُ أَحَدٌ يُرَدِّدُهَا فَلَمَّا أَصْبَحَ جَاءَ إِلَى النَّبِيِّ ﷺ فَذَكَرَ ذَلِكَ لَهُ فَقَالَ رَسُولُ اللهِ ﷺ «وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ»  

"Qul Huwa Allahu ahad [Say: He is Allah, (the) One] is equivalent to one third of the Quran." (Using translation from Ibn Mājah 3787)   

عَنِ النَّبِيِّ ﷺ قَالَ «قُلْ هُوَ اللهُ أَحَدٌ ثُلُثُ الْقُرْآنِ»