Reference: Fataawa Ramadhaan – Volume 2, Page 518, Fatwa No.482
al-Fataawa libni-‘Uthaymeen – Kitaab ad-Da’wah, Volume 1, Page 161-167
Question: Is the fast of one who gave false witness correct?
Response: Giving false witness is from the most major sins. That is, for someone to bear witness to that which he does not know or that which he knows opposite to. This does not invalidate the fast but does reduce it’s reward.