It is not permissible to testify to anything except that which you are certain about and know. You are not permitted to give an untrue statement or testify to something about which you have no information.
This is considered false testimony. Allah (Exalted be He) says:
...and shun lying speech (false statements) —
In the Hadith
agreed upon by Al-Bukhari and Muslim,
after the Messenger of Allah (peace be upon him) mentioned some major sins, he (peace be upon him) added:
'And I warn you against false speech and false testimony. I warn you against false speech and false testimony.' The Prophet (peace be upon him) kept on saying that warning until the Sahabah (Companions of the Prophet) wished that he would stop.
That is, out of awe of the gravity of this major sin. (Part No. 23; Page No. 516) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.