Sayyiduna_ Abu Hurayrah
رضى الله تعالى عنه
narrates that
رسول الله صلى الله تعالى عليه وسلم
said:
That person who does not abstain from speaking lies and acting upon it, certainly ALLAH TA'ALA does not have any need, in that he leaves out his food and his drink
(Bukhari)
There is no divine reward for the fasting person, who indulges in lies, even though he will be absolved of the compulsion of the fast.
Every sinful utterance is included, such as false testimony, backbiting, slandering, vulgar language, futile conversation, arguing, etc.
Basically, we need to abstain from everything ALLAH TA'ALA has forbidden.