Reference: Fataawa Noor ‘alad-Darb – Question No.6036, Page 453, Volume 11
Question: In the presence of my friends, sisters and teachers, I swear an oath based on a lie, whilst [at the same time] in my heart I seek forgiveness from Allaah by oft-repeating al-istighfaar – without anyone knowing about it; am I still sinful for doing so?
Response: Yes, you are sinful in that. It is impermissible to lie, so how about attaching that lie to an oath – which would thereby increase the severity of the sin. Some of the scholars have even said if someone swears an oath based upon a lie – knowing it to be a lie, then that act in itself is referred to as “the false oath” which immerses the perpetrator in sin as well as the Fire [of Hell].