Sayyiduna Abdullah bin Sarjis says: I submitted to the Prophet (peace and blessings be upon him): I love Sayyiduna Abu Dharr al-Ghifari (may Allah be pleased with him). The Prophet (peace and blessings be upon him) asked: Have you told him this? I replied: No. The Prophet (peace and blessings be upon him) said: Tell him! The narrator says: I met Sayyiduna Abu Dharr al-Ghifari (may Allah be pleased with him) and said: I love you for the sake of Allah Almighty. He said: May the One for whose sake you love me love you too. The narrator says: I went back to the Prophet (peace and blessings be upon him) and informed him about this, and the Prophet (peace and blessings be upon him) said: "Whoever mentions it (that is, whoever tells another, 'I love you for the sake of Allah Almighty') will receive a reward."
Imam al-Tabarani recorded it, and its chain contains narrators whom I do not know.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الزهد / 18034