It was narrated from Aishah that: 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."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا سُلَيْمَانُ بْنُ دَاوُدَ، عَنِ ابْنِ وَهْبٍ، قَالَ حَدَّثَنَا عَمْرُو بْنُ الْحَارِثِ، عَنْ سَعِيدِ بْنِ أَبِي هِلاَلٍ، أَنَّ أَبَا الرِّجَالِ، مُحَمَّدَ بْنَ عَبْدِ الرَّحْمَنِ حَدَّثَهُ عَنْ أُمِّهِ، عَمْرَةَ عَنْ عَائِشَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم بَعَثَ رَجُلاً عَلَى سَرِيَّةٍ فَكَانَ يَقْرَأُ لأَصْحَابِهِ فِي صَلاَتِهِمْ فَيَخْتِمُ بِـ { قُلْ هُوَ اللَّهُ أَحَدٌ } فَلَمَّا رَجَعُوا ذَكَرُوا ذَلِكَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ " سَلُوهُ لأَىِّ شَىْءٍ فَعَلَ ذَلِكَ " . فَسَأَلُوهُ فَقَالَ لأَنَّهَا صِفَةُ الرَّحْمَنِ عَزَّ وَجَلَّ فَأَنَا أُحِبُّ أَنْ أَقْرَأَ بِهَا . قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَخْبِرُوهُ أَنَّ اللَّهَ عَزَّ وَجَلَّ يُحِبُّهُ " .
sunnah.com