Narrated Anas ibn Malik:

A man was with the Prophet ﷺ and a man passed by him and said: Messenger of Allah! I love this man. The Messenger of Allah ﷺ then asked: Have you informed him? He replied: No. He said: Inform him. He then went to him and said: I love you for Allah's sake. He replied: May He for Whose sake you love me love you!

حَدَّثَنَا مُسْلِمُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا الْمُبَارَكُ بْنُ فَضَالَةَ، حَدَّثَنَا ثَابِتٌ الْبُنَانِيُّ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّ رَجُلاً، كَانَ عِنْدَ النَّبِيِّ ﷺ فَمَرَّ بِهِ رَجُلٌ فَقَالَ يَا رَسُولَ اللَّهِ إِنِّي لأُحِبُّ هَذَا . فَقَالَ لَهُ النَّبِيُّ ﷺ " أَعْلَمْتَهُ " . قَالَ لاَ قَالَ " أَعْلِمْهُ " . قَالَ فَلَحِقَهُ فَقَالَ إِنِّي أُحِبُّكَ فِي اللَّهِ . فَقَالَ أَحَبَّكَ الَّذِي أَحْبَبْتَنِي لَهُ .