'A'isha reported:The Messenger of Allah (ﷺ) sent a man in charge of an expedition and he would recite for his Companions during their prayer, ending (recitation) with:" Say, He is God, One." When they returned mention was made of it to the Messenger of Allah (ﷺ). He (the Holy Prophet) told them to ask him why he had done like that. So they asked him and he said: Verily, it is an attribute of the Compassionate One, and (for this reason) I love to recite it. The Messenger of Allah (ﷺ) thereupon said: Inform him that Allah loves him.
حَدَّثَنَا أَحْمَدُ بْنُ عَبْدِ الرَّحْمَنِ بْنِ وَهْبٍ، حَدَّثَنَا عَمِّي عَبْدُ اللَّهِ بْنُ وَهْبٍ، حَدَّثَنَا عَمْرُو بْنُ الْحَارِثِ، عَنْ سَعِيدِ بْنِ أَبِي هِلاَلٍ، أَنَّ أَبَا الرِّجَالِ، مُحَمَّدَ بْنَ عَبْدِ الرَّحْمَنِ حَدَّثَهُ عَنْ أُمِّهِ، عَمْرَةَ بِنْتِ عَبْدِ الرَّحْمَنِ وَكَانَتْ فِي حَجْرِ عَائِشَةَ زَوْجِ النَّبِيِّ صلى الله عليه وسلم عَنْ عَائِشَةَ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم بَعَثَ رَجُلاً عَلَى سَرِيَّةٍ وَكَانَ يَقْرَأُ لأَصْحَابِهِ فِي صَلاَتِهِمْ فَيَخْتِمُ بِـ { قُلْ هُوَ اللَّهُ أَحَدٌ} فَلَمَّا رَجَعُوا ذُكِرَ ذَلِكَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ " سَلُوهُ لأَىِّ شَىْءٍ يَصْنَعُ ذَلِكَ " . فَسَأَلُوهُ فَقَالَ لأَنَّهَا صِفَةُ الرَّحْمَنِ فَأَنَا أُحِبُّ أَنْ أَقْرَأَ بِهَا . فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَخْبِرُوهُ أَنَّ اللَّهَ يُحِبُّهُ ".
Muslim
The Book of Prayer - Travellers • Hadith 318