Ayesha reported,
The Messenger of Allah ﷺ sent a man in command of a military expedition. He used to lead his companions in prayer and would always conclude his recitation with "Say, He is Allah, the One." When they returned, they mentioned this to the Messenger of Allah ﷺ.He said, "Ask him why he did that."So they asked him, and he replied, "Because it describes the Most Merciful 
, and I love to recite it."The Messenger of Allah ﷺ said, "Tell him that Allah loves him."
Nasai 994
Sahihأخبرنا سليمان بن داود عن ابن وهب قال حدثنا عمرو بن الحارث عن سعيد بن أبي هلال أن أبا الرجال محمد بن عبد الرحمن حدثه عن أمه عمرة
عن عائشة
أن رسول الله ﷺ بعث رجلا على سرية فكان يقرأ لأصحابه في صلاتهم فيختم بقل هو الله أحد فلما رجعوا ذكروا ذلك لرسول الله ﷺ فقال سلوه لأي شيء فعل ذلك فسألوه فقال لأنها صفة الرحمن عز وجل فأنا أحب أن أقرأ بها قال رسول الله ﷺ أخبروه أن الله عز وجل يحبه
Bukhari transmitted it in the Book of Tawhid, in the chapter on what has been mentioned regarding the Prophet's ﷺ call to his nation to affirm the oneness of Allah 
, hadith no. 7375, and Muslim in the Book of the Prayer of Travelers, in the chapter on the merit of reciting "Say, He is Allah, the One," hadith no. 813, from the hadith of Ibn Wahb through the same chain. It is also found in al-Kubra, hadith no. 1065.