It is narrated from Sayyiduna Mughirah bin Shu'bah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever narrates a hadith from me while he considers it to be a lie, he is one of the liars."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18146