All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
You are not blamed for hearing backbiting while praying because you did not intend to listen to it, and you were not in that meeting.
The scholars stated that the listener to backbiting unintentionally is not sinful.
Shaykh Ibn Taymiyyah said, “If a person hears Kufr (disbelief), lying, backbiting, singing, and music unintentionally; rather, he only heard that as he was passing by, then he is not sinful, by the agreement of all the Muslims. However, if he sits down and listens to that without denying it with his heart or tongue or hand, then he is sinful, with the agreement of all the Muslims.”
Backbiting is one of the major sins. Al-Haytami, when talking about it in Az-Zawaajir, said, “It appears from the many pieces of authentic evidence that it is a major sin, but its seriousness differs according to the difference of the harm which it leads to.”
Regarding denying evil, please refer to fatwas 350461 and 363466.
Allah knows best.