Reference : Hisn al-Muslim 224

قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم: (لاَ تَدْخُلُوا الْجَنَّةَ حَتَّى تُؤْمِنُوا، وَلاَ تُؤْمِنُوا حَتَّى تَحَابُّوا، أَوَلاَ أَدُلُّكُم عَلَى شَيْءٍ إِذَا فَعَلْتُمُوهُ تَحَابَبْتُم، أَفْشُوا السَّلاَمَ بَيْنَكُمْ)
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The Prophet (ﷺ) said: "You shall not enter Paradise until you believe, and you have not believed until you love one another. Shall I tell you of something you can do to make you love one another? Spread the greetings of Salam (peace) amongst yourselves (i.e. between each other).

Reference:
Muslim 1/74 and others.

Chapter: Excellence of spreading the Islamic greeting