It was narrated that Abu Hurairah said:
"The Messenger of Allah (ﷺ) said: 'By the One in whose Hand is my soul! You will not enter Paradise until you believe, and you will not (truly) believe until you love one another. Shall I not tell you of something which, if you do it, you will love one another? Spread the greetings of Salam amongst yourselves.'"
English reference: Vol. 1, Book 1, Hadith 68
Arabic reference: Book 1, Hadith 71