The Prophet ﷺ listed false testimony among the very greatest of major sins alongside associating partners with Allah and disobedience to parents, and he kept repeating the warning until the companions wished he would stop, out of concern for how it distressed him (Sahih al-Bukhari 2654). Allah commands believers to stand firmly for justice as witnesses for Allah even against themselves, their parents and their relatives (Quran 4:135), and He describes the servants of the Most Merciful as those who do not bear witness to falsehood (Quran 25:72). This covers lying in court, signing a false statement, supporting a relative's fraudulent insurance, benefits or immigration claim, and giving a knowingly dishonest reference or character statement. Concealing testimony is also sinful — "do not conceal testimony, for whoever conceals it, his heart is sinful" (Quran 2:283) — so staying silent when your word would prevent an injustice, without a valid excuse, is itself a wrong.
Q&A · Society & Ethics
What is the ruling on giving false testimony or backing someone's false claim?
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.