Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) narrates that he heard the Prophet (peace and blessings be upon him) say: "You will not be believers until you love one another. Shall I not guide you to that by which you will love one another?" The people said: "Yes, O Messenger of Allah!" The Prophet (peace and blessings be upon him) said: "Spread the greeting of peace (salam) among yourselves. By the One in whose hand is my soul, you will not enter Paradise until you show mercy to one another." The people said: "Yes, we are all merciful." The Prophet (peace and blessings be upon him) said: "What is meant is not that one of you shows mercy to his companion; rather, general mercy (to all) is meant."
Al-Tabarani recorded it. Its chain contains Abdullah bin Salih, who was declared trustworthy, while a group declared him da'if (weak). This hadith also has a route in the Book of Repentance.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12731