Sayyiduna Ismah (may Allah be pleased with him) reports: One day Sayyiduna Abbas bin Abd al-Muttalib (may Allah be pleased with him) entered the mosque and greeted the people with salam, and he sensed displeasure on the people's faces. So he went to the Prophet (peace and blessings be upon him) at his house and said: O Messenger of Allah! What is the reason that when I entered the mosque, I sensed displeasure on the people's faces? The narrator says: The Prophet (peace and blessings be upon him) came out, entered the mosque and said: "O assembly of people! You will not be counted as believing in Allah Almighty, nor will you be counted as believers, until you love Sayyiduna Abbas (may Allah be pleased with him)."
Al-Tabarani recorded it; its chain contains a narrator, al-Fadl bin al-Mukhtar, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 15475