Authentic Supplication Number 217.

The Messenger of Allah  said, "You shall not enter Heaven (Al-Jennah) till you have faith, and you do not have faith till you love one another, shall I not guide you upon something if you do it you wil love one another? Spread (231) As-Salam among you."

Footnotes.
(231) Meaning greet one another with this greeting. It is a supplication for the peace, mercy, and blessings of Allah.

Reporters.
Reported by Muslim, and is #154 in The Authentic of Good Sayings.