As-Salaam alaikum,
Abu Hurayra, may Allah be pleased with him, reported that the Prophet, Sallallahu alaihi Wasallam, said:--

"By He in whose hand my soul rests! None of you will enter
Paradise until you truly believe, and none of you will truly
believe until you love one another. Shall I indicate to you
how you might love one another? Extend your greetings
amongst each other far and wide."
(Abu Dawud, Muslim, al-Tirmidhi and Ibn Majah).

Abu Hurayra, may Allah be pleased with him, reported:--

"O Messenger of Allah, what is character assassination (ghiba)?". He replied,
'It is your act of mentioning your brother in a manner he despises.'
[The interlocutor] said, 'But what if your brother matches the description
that you have mentioned?'

[The Prophet] replied: 'If he is as you describe him, then you have committed
character assassination; if he is not as you describe him, you have committed
slander."
(Abu Dawud, Muslim, Tirmidhi).