“And follow not, (O man i.e. do not say, act or witness) that of which you have no knowledge. Verily! The hearing, and the sight and the heart of each of those ones will be questioned.” [Al-Israa (17):36]
And He also warns us not to speak about Him without knowledge:
“…and saying things about Allah of which you have no knowledge.” [Al-A’araaf (7):33]
In the above Ayah, Allah is telling us that He has forbidden Muslims to speak about Him without knowledge, and lying upon Allah is one way of doing this. The one who lies about Allah is not only guilty of speaking about Him without knowledge, but also he is amongst the most oppressive of people as Allah Himself describes:
“And who is more oppressive than he who lies against Allah, or who denies His signs…”
Besides that, He, the Wise, also describes the punishment, on the Day of Judgment, for anyone who involves themselves in this dangerous practice, thus:
“And you will see on the Day of Judgment, those who lie upon Allah, their faces blackened.” [Az-Zumar (39): 60]
Many Muslims would tell you that stealing, bribery, killing, adultery etc. are all prohibited (Haram) acts. However, when it comes to lying upon Allah and/ or His Messenger they are not even aware that such a sin exists, and so they would indulge in it freely without worry. This complacency among Muslims regarding this sin comes about because of the great ignorance that prevails concerning
1) the rights of Allah and His Messenger and
2) the serious punishment that results from indulging in sin.
This complacent attitude, coupled with ignorance, has been the main factors for the majority of the deviations and differences found in Islam.