Sayyidena Abu Saeed Khudri and Jabir (Allah be pleased with them) narrate that the Prophet (Allah bless him and give him peace) said,

‘Backbiting is severe and worst than adultery (zina). ‘

Some of the holy Companions inquired why was the severity of backbiting worst than adultery.

The Prophet (Allah bless him and give him peace) replied,

‘If an individual commits adultery due to his wretched fate than if he repents it will be forgiven by Allah.
Whereas, for backbiting it is required to ask forgiveness from the individual who’s backbiting was done. It will not be forgiven or condoned by Allah.

Sahih Muslim via Uswa e Rasool e Akram (Allah bless him and give him peace), page 626