Abu Hurairah (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "By Him in Whose Hand is my life! You will not enter Jannah until you believe, and you will not believe until you love one another. Shall I inform you of something which, if you do, you will love one another? Promote greetings amongst yourselves."[Muslim].
App reference: Book 6, Hadith 4
Arabic/English book reference: Book 6, Hadith 848