Rasulallah (saw) is reported to have said: Gheebat is worse than zina (adultery)?
Can you give me the reference of this hadith?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

This Hadeeth is reported by Al-Bayhaqi in “Shu’ab Al-Eemaan”, and it was also reported by Ibn Abi Ad-Dunya that the Prophet said: "Beware of Gheebah (backbiting), indeed backbiting is worse than Zina (fornication and/or adultery); a person may commit Zina and Allaah accepts his repentance, but Allaah does not forgive the backbiting until the person whom he backbit forgives him.” This Hadeeth is Dha’eef [weak] as stated by Shaykh Al-Albaani in “Dha’eef Al-Jaami’”.

Moreover, Al-Haythami stated in “Majma’ Az-Zawaa’id” that in its chain of narrators there is ‘Abbaad ibn Katheer Ath-Thaqafi and he is Matrook (abandoned) and the author of “Kashf Al-Khafaa’” said: “As-Saaghaani said that it (this Hadeeth) is fabricated.”

Besides, Ibn Abi Haatim quoted this Hadeeth in his book entitled “Al-‘Ilal” and he said: “I said to my father (Abu Haatim): this Hadeeth is Munkar (denounced), and he answered: “It is as you said.”

Allaah Knows best.