These words have been reported as a Hadith with several very weak chains. Imam Tabarani (rahimahullah) has recorded this narration. However, ‘Allamah Haythami (rahimahullah) has declared a narrator extremely weak. (Al Mu’jamul Awsat, Hadith: 6586, Majma’uz Zawaid, vol. 8 pg. 91/92).  The learned scholar Ibn Hajar al-Haytimi has said (in his book Fataweh al-Fiqhiyeh Al-Kubrah), “…with regard to the narration ‘Backbiting is worse than fornicating thirty times while Muslim’, I have not seen it mentioned in the books of Hadith, both the extensive and abridged versions, and so it appears that there is no origin to this saying.”

It is therefore not suitable to quote this statement as a Hadith of Nabi (sallallahu ‘alayhi wa sallam). 

Imam Bayhaqi (rahimahullah) has recorded these words in Shu’abul Iman as the statement of Sufyan ibn ‘Uyaynah (rahimahullah). Hafiz Mundhiri (rahimahullah) says that this is more correct. (Targhib, vol. 3 pg. 511)

The complete narration is as follows: Sufyan ibn ‘Uyaynah (rahimahullah) says: “In the sight of Allah, backbiting is worse than  adultery and consuming alcohol because adultery and consuming alcohol is between you and Allah, if you repent, Allah will forgive you. However backbiting is never forgiven until the person you spoke against forgives you”. (Shu’abul Iman, Hadith: 6314)

This narration is suitable to be quoted as the statement of Sufyan ibn ‘Uyaynah (rahimahullah) and not as a Hadith of Nabi (sallallahu ‘alayhi wa sallam).

 And Allah Ta’ala Knows best

https://hadithanswers.com/another-narration-regarding-ghibah-and-adultery/

https://hadithanswers.com/verification-of-hadith-backbiting-is-worse-than-adultery/

https://islamqa.org/shafii/qibla-shafii/33096/backbiting-is-worse-than-fornication/