Hadith: I love you for Allah's sake. He said: May the One you love me for His sake love you

عن أنس بن مالك رضي الله عنه أنَّ رَجُلاً كَانَ عِند النَبِيَّ صلى الله عليه وسلم فَمَرَّ رَجُلٌ بِهِ، فقال: يا رسول الله، أنِّي لَأُحِبُّ هَذَا، فَقَال لَهُ النَبِيُّ صلى الله عليه وسلم : «أَأَعْلَمْتَهُ؟» قال: لا. قال: «أَعْلِمْهُ»، فَلَحِقَهُ، فقال: إِنِّي أُحِبُّك فِي الله، فقال: أّحَبَّك الَّذِي أَحْبَبْتَنِي لَهُ.
[صحيح] - [رواه أبو داود والنسائي في الكبرى وأحمد]

Anas ibn Mālik (may Allah be pleased with him) reported that a man was with the Prophet (may Allah's peace and blessings be upon him) and another man passed by him. He said: "O Messenger of Allah, I love this man." The Prophet (may Allah's peace and blessings be upon him) said: "Have you told him?" He said: 'No!' He said: "Tell him." He went after him and said: "I love you for Allah's sake." He said: "May the One you love me for His sake love you!"
[Authentic hadith] - [An-Nasaa’i - Narrated by Abu Daoud - Narrated by Ahmad]

Explanation

This Hadīth comes as an implementation of the command of the Prophet (may Allah's peace and blessings be upon him) for one to inform his brother if he loves him. When a man was sitting with him said that he loved "that man", i.e. another man who was passing them by, the Prophet (may Allah's peace and blessings be upon him) said: "Have you told him?" This indicates that the Sunnah institutes saying "I love you" to anyone one loves, because this word puts love in his heart, as if he knows that his brother loves him, he will love him back, even though the hearts sense love even if not expressed by words. Similarly, the Prophet (may Allah's peace and blessings be upon him) said: "The souls are troops collected together. Those of them who are familiar with one another unite, and those who are unfamiliar with each other are in conflict." However, when one expresses his love in words, saying "I love you for Allah's sake", it will increase his brother's love for him.