Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) appointed a man in charge of an army unit who led them in prayer; he always concluded his recitation with Sūrat al-Ikhlās

عن عائشة رضي الله عنها «أن رسول الله صلى الله عليه وسلم بعث رجلا على سَرِيَّةٍ فكان يَقْرَأ لأصحابه في صلاتهم، فَيَخْتِمُ بـ«قل هو الله أحد» فلما رجعوا ذكروا ذلك لرسول الله صلى الله عليه وسلم فقال: سَلُوهُ لأَيِّ شيء صَنَع ذلك؟ فسألُوه، فقال: لِأنَّها صِفَةُ الرحمن عز وجل ، فأنا أُحِب أَنْ أَقْرَأ بها، فقال رسول الله صلى الله عليه وسلم : أَخْبِرُوه: أنَّ الله تعالى يُحِبُّه».
[صحيح] - [متفق عليه]

‘Ā’ishah (may Allah be pleased with her) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) appointed a man in charge of an army unit who led them in prayer; he always concluded his recitation with Sūrat al-Ikhlās. Upon their return to Madīnah, they mentioned this to the Messenger of Allah (may Allah's peace and blessings be upon him), who said: "Ask him why he did so?" He was asked, and he responded: "This Sūrah contains the Attributes of Allah, the Gracious, and I love to recite it." The Messenger of Allah (may Allah's peace and blessings be upon him) then told them: "Tell him that Allah loves him."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) appointed a man in charge of an army unit to be in charge and judge between them so that everything take place in an orderly fashion and without chaos. He chose the most religious, knowledgeable, and expert one of them. For this reason, rulers and scholars were to lead the prayers due to their knowledge and religion. He always concluded his recitation with Sūrat al-Ikhlās in the second Rak‘ah of every prayer because of his love of Allah and His names and attributes. It is natural whenever someone loves something that he increases its remembrance. When they returned to Madīnah, they reported this to the Prophet. So he said: "Ask him why he does this!" Is it a coincidence, or is there a special reason? The leader said: "I do this because this Sūrah contains the attributes of the Most Merciful and I love to repeat it because of this. So the Prophet (may Allah's peace and blessings be upon him) said: "Inform him, just as he repeats this Sūrah - because of what it contains of Allah's beautiful names and attributes and because he loves it - Allah loves him as well."