In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Allaah [The Most High] said:
ذَٰلِكَ وَمَنْ يُعَظِّمْ حُرُمَاتِ اللَّهِ فَهُوَ خَيْرٌ لَهُ عِنْدَ رَبِّهِ ۗ وَأُحِلَّتْ لَكُمُ الْأَنْعَامُ إِلَّا مَا يُتْلَىٰ عَلَيْكُمْ ۖ فَاجْتَنِبُوا الرِّجْسَ مِنَ الْأَوْثَانِ وَاجْتَنِبُوا قَوْلَ الزُّورِ
That (Manasik prescribed duties of Hajj is the obligation that mankind owes to Allah), and whoever honours the sacred things of Allah, then that is better for him with his Lord. The cattle are lawful to you, except those (that will be) mentioned to you (as exceptions). So, shun the abomination (worshipping) idol, and shun Qawl Az-Zoor) [Surah Al-Hajj’ Aayah 30]
[فَاجْتَنِبُوا الرِّجْسَ مِنَ الْأَوْثَانِ وَاجْتَنِبُوا قَوْلَ الزُّورِ – So shun the abomination [worshipping] idol, and shun Qawl Az-Zoor]- Meaning: All forbidden speech, for indeed it is from Qawl Az-Zoor- lies and false witness. [Ref 1]
False witness is among the greatest of Major sins. The Prophet [sallal-laahu-alayhi-wasallam] said: Should I not inform you about the greatest of Major the Major sins? We [i.e. the Sahaabah] said, ‘’Certainly O Messenger of Allaah! He said, ‘’Associating partners with Allaah and disobedience to parents.’’ The Prophet was reclining, then he sat up and said, ‘’And false witness’’. He did not stop repeating it [i.e. warning against false witness] until we said, ‘’If only he stops’’ [i.e. stops repeating it in order not to bring difficulty upon himself] [End of hadeeth]
Even though Shirk is the greatest of all sins and crimes, however false witness is a great crime that is connected to great evil and oppression against the people. It is obligated on a male believer and a female believer to beware of [false witness]. A person should only witness to the truth – be it on behalf of a friend or enemy – based on what he [or she] knows, just as Allaah [The Most high] said: [إِلَّا مَن شَهِدَ بِالْحَقِّ وَهُمْ يَعْلَمُونَ – except those who bear witness to the truth and they know (the facts) [43:86].
A person must know what he is bearing witness to; but if not he [or she] should fear Allaah and does should not bear witness except to that which he [or she] knows – even if the one he [or she] bears witness for is a friend, relative, father, uncle or other than them. It is obligated on a person to fear Allaah and should not bear witness except to that which he [or she] knows. [Ref 2]
Ref 1: An Excerpt from Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer kalaam al-Mannaan. Abridged and paraphrased
Ref 2. By Imaam Abdul Azeez Bin Baaz [rahimahullaah]. See link: http://www.sahab.net/forums/?showtopic=137278