The Prophet ﷺ said, "When one of you loves his brother, let him tell him that he loves him."
حدثنا يحيى بن سعيد قال حدثنا ثور يعني ابن يزيد قال حدثني حبيب بن عبيد
عن النبي ﷺ قال إذا أحب أحدكم أخاه فليعلمه أنه يحببه
Its isnad is sahih, and Habib bin Ubayd is Rahbi, Abu Hafs of Homs, a trustworthy narrator known to be reliable. His hadith appears in Muslim and the four Sunan, and Bukhari transmitted from him in al-Adab. This hadith was narrated by Abu Dawud, 4/332, no. 5124, in al-Adab, on informing a man that one loves him, and by Bukhari in al-Adab, 189, no. 542, and Ibn Hibban, 623, no. 2514 in al-Mawarid. Hakim graded it sahih, 4/171, and Dhahabi agreed with him.