Volume 3, Book 48, Number 821 :
Narrated by Anas (radiallaahu `anhu)
The Prophet (sallallaahu `alayhi wasallam) was asked about the great sins He said, “They are:- (1 ) To join others in worship with Allah, (2) To be undutiful to one’s parents. (3) To kill a person (which Allah has forbidden to kill) (i.e. to commit the crime of murdering). (4) And to give a false witness.”
Volume 3, Book 48, Number 822 :
Narrated by Abu Bakra (radiallaahu `anhu)
The Prophet (sallallaahu `alayhi wasallam) said thrice, “Should I inform you out the greatest of the great sins?” They said, “Yes, O Allah’s Apostle (sallallaahu `alayhi wasallam)!” He said, “To join others in worship with Allah and to be undutiful to one’s parents.” The Prophet (sallallaahu `alayhi wasallam) then sat up after he had been reclining (on a pillow) and said, “And I warn you against giving a false witness”, and he kept on saying that warning till we thought he would not stop.
There’s mention of false witness in the Quran as well:
وَالَّذِينَ لَا يَشْهَدُونَ الزُّورَ
“And [they are] those who do not testify to falsehood..” [Al-Furqan: 72]
Ibn Kathir comments:
These are further attributes of the servants of the Most Gracious. They do not bear witness to falsehood, including lies, immorality, disbelief, foul speech and false words…
“And those who do not bear witness to falsehood” refers to giving false testimony, which means lying deliberately to someone else…
From the context it seems that what is meant by those who do not bear witness to falsehood is those who do not attend it or are not present when it happens.
The above ayah talks about false witness, which is a major sin. Allah also talks about hiding witness/testimony in the Quran:
وَلَا تَكْتُمُوا الشَّهَادَةَ ۚ وَمَن يَكْتُمْهَا فَإِنَّهُ آثِمٌ قَلْبُهُ ۗ وَاللَّهُ بِمَا تَعْمَلُونَ عَلِيمٌ
“And do not conceal testimony, for whoever conceals it – his heart is indeed sinful, and Allah is Knowing of what you do.” [Al-Baqarah: 283]
Ibn Kathir comments:
﴿وَلاَ تَكْتُمُواْ الشَّهَـدَةَ﴾
(And conceal not the evidence) means, do not hide it or refuse to announce it. Ibn `Abbas and other scholars said, “False testimony is one of the worst of the major sins, and such is the case with hiding the true testimony.” This is why Allah said,
﴿وَمَن يَكْتُمْهَا فَإِنَّهُ ءَاثِمٌ قَلْبُهُ﴾
(For he who hides it, surely, his heart is sinful).
As-Suddi commented, “Meaning he is a sinner in his heart.”
This is similar to Allah’s statement,
﴿وَلاَ نَكْتُمُ شَهَـدَةَ اللَّهِ إِنَّآ إِذَاً لَّمِنَ الاٌّثِمِينَ﴾
(We shall not hide testimony of Allah, for then indeed we should be of the sinful) ﴿5:106﴾.
﴿يَـأَيُّهَا الَّذِينَ ءَامَنُواْ كُونُواْ قَوَّامِينَ بِالْقِسْطِ شُهَدَآءِ للَّهِ وَلَوْ عَلَى أَنفُسِكُمْ أَوِ الْوَلِدَيْنِ وَالاٌّقْرَبِينَ إِن يَكُنْ غَنِيّاً أَوْ فَقَيراً فَاللَّهُ أَوْلَى بِهِمَا فَلاَ تَتَّبِعُواْ الْهَوَى أَن تَعْدِلُواْ وَإِن تَلْوُواْ أَوْ تُعْرِضُواْ فَإِنَّ اللَّهَ كَانَ بِمَا تَعْمَلُونَ خَبِيراً ﴾
(O you who believe! Stand out firmly for justice, as witnesses to Allah, even though it be against yourselves, or your parents, or your kin, be he rich or poor, Allah is a better Protector to both (than you). So follow not the lusts (of your hearts), lest you avoid justice; and if you distort your witness or refuse to give it, verily, Allah is Ever Well-Acquainted with what you do) ﴿ 4:135
P.S. We have talked previously about a similar subject: false oaths.