Mukhtasar Bukhari (Abridged) 2220 — The Prophet’s call to the oneness

Narrated from Ummul Momineen Aisha (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) appointed a man as the leader of a military expedition. When that man led the prayer, he would always complete his recitation with (Qul Huwa Allahu Ahad). When they returned, they mentioned this to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) said: "Ask him why he does this." The people asked him, and he said (that I recite it frequently because) it describes the attributes of the Most Merciful (i.e., Allah), and I love to recite it. The Prophet (peace and blessings be upon him) said: "Tell him that Allah loves him."