If he has uttered it, then he must fulfill it.

I read that before, but I am a bit unclear about something. If a person says a vow like “I vow to fast 20 days”, but he did not intend to actually vow, he just said it as a lie or showoff, then is it considered as a valid vow? Does he have to fulfill it as it is a vow to do an act of worship, even though he did not intend to vow?