It is narrated from Sayyiduna Samurah bin Jundub (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever narrates a hadith from me while he thinks that it is a lie, then he too is one of the liars."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6727