It was narrated from Anas ibn Maalik (may Allaah be pleased with him) that a man was with the Prophet (peace and blessings of Allaah be upon him) when another man passed by and he said: O Messenger of Allaah, I love this man. The Prophet (peace and blessings of Allaah be upon him) said to him: “Have you told him?” He said: No. He said: “Tell him.” So he caught up with him and said: I love you for the sake of Allaah. He said: May the one for Whose sake you love me also love you.
Narrated by Abu Dawood (no. 5125) and classed as saheeh by al-Nawawi in Riyadh al-Saaliheen (183) and classed as hasan by al-Albaani in Saheeh Abi Dawood.