The Messenger of Allah ﷺ took him by the hand and said, "Muaz, by Allah, I love you. By Allah, I love you."Then he said, "I advise you, Muaz, never to omit saying at the end of every prayer, 'O Allah, help me to remember You, to thank You, and to worship You well.'"
أن رسول الله ﷺ أخذ بيده وقال: يا معاذ والله إني لأحبك، والله إني لأحبك فقال: أوصيك يا معاذ لا تدعن في دبر كل صلاة تقول: اللهم أعني على ذكرك وشكرك وحسن عبادتك.
Reported by Abu Dawud (1522), and this is his wording, and Nasai (1303), both through the route of Haywa bin Shurayh, who said, I heard Uqba bin Muslim say, Abu Abdul-Rahman Hubli narrated to me, from Sanabihi, from Muaz bin Jabal, and he related it.Nasai added, after the Prophet's ﷺ words to Muaz, "By Allah, I love you," that Muaz said, "And I love you too, Messenger of Allah."Abu Dawud said, Muaz enjoined this upon Sanabihi, and Sanabihi enjoined it upon Abu Abdul-Rahman. Nasai added, in Al-Yawm wal-Laylah (109), that Abu Abdul-Rahman enjoined it upon Uqba bin Muslim.Its chain of transmission is sahih, and Nawawi graded it sahih in Al-Adhkar (201), placing it after the prayer.