Sayyiduna Ismah (may Allah be pleased with him) said: 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 saw dislike on their faces. So he went back to the Messenger of Allah (peace and blessings be upon him) in his house and said: "O Messenger of Allah, what is the matter with me, that whenever I enter the mosque I see dislike on the people's faces?" So the Messenger of Allah (peace and blessings be upon him) came, entered the mosque and said: "O people, you will never have faith and you will never be believers until you love Abbas."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14566