Qad naAAlamu innahu layahzunuka allathee yaqooloona fa-innahum la yukaththiboonaka walakinna aththalimeena bi-ayati (surah Anam: 33)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(6:33) (O Muhammad!) We know indeed that the things they say grieve you, though in truth it is not you whom they give the lie to, but it is the signs of Allah that these wrong-doers reject *21.

Qad na`lamu innahoo layahzunukal lazee yaqooloona fa innahum laa yukazziboonaka wa laakinnaz zaalimeena bi Aayaatil laahi yajhadoon

*21). The fact is that before the Prophet Muhammad (peace be on him) began to preach the message of God, all his people regarded him as truthful and trustworthy and had full confidence in his veracity. Only after he had begun to preach the message of God did they call him a liar. Even during this period none dared to say that the Prophet (peace be on him) had ever been guilty of untruthfullness in personal matters. Even his worst enemies never accused him of lying in any worldly affairs. When they did accuse him of falsehood, they did so in respect of his prophetic mission. Abu Jahl for instance, was one of his staunchest enemies. According to a tradition from 'Ali, Abu Jahl once said to the Prophet (peace be on him): 'We do not disbelieve you. We disbelieve your message.' On the occasion of the Battle of Badr, Akhnas b. Shariq asked Abu Jahl, when they were alone, to confide whether he considered Muhammad to be truthful or not. He replied: ' By God, Muhammad is a veracious person. He has never lied in all his life. But if every honorable office-liwa' (standard-bearing in war), siqayah ( provision of water to the pilgrims), hijabah (gaurdian ship of the ka'bah ) and nubuwah ( prophethood ) - were to fall to the share of the decendents of Qusayy, what would be left for the rest of the Quraysh?'* (Ibn Kathir,vol.3, pp.17-18-Ed.)
Here God consoled the Prophet(peace be on him) by telling him that by charging him with falsehood the unbelievers were calling God untruthful. Since God has endured this accusation with mild forbearance, leaving them free to persist in their blasphemy, the Prophet ( peace be on him ) need not feel undue disquiet.