A man was sitting with the Prophet ﷺ when another man passed by. He said, "Messenger of Allah, I truly love this man."The Prophet ﷺ asked him, "Have you told him?"He replied, "No."The Prophet ﷺ said, "Tell him."So he caught up with him and said, "I love you for the sake of Allah."The other man replied, "May the One for whose sake you love me love you."
حدثنا مسلم بن إبراهيم حدثنا المبارك بن فضالة حدثنا ثابت البناني
أن رجلا كان عند النبي ﷺ فمر به رجل فقال يا رسول الله إني لأحب هذا فقال له النبي ﷺ أعلمته قال لا قال أعلمه قال فلحقه فقال إني أحبك في الله فقال أحبك الذي أحببتني له
[Its isnad is hasan] Ahmad reported it, 3/150, from the hadith of Mubarak bin Fadala, and Nasai in al-Kubra, no. 10010, and Amal al-Yawm wal-Layla, no. 182, from the hadith of Thabit Bunani through him, and Nawawi graded it sahih in Riyad al-Salihin, no. 386.