Musnad Ahmad 626:78

Waki' narrated to us, he said: Sufyan and Shu'bah narrated to us, from Habib ibn Abi Thabit, from Maymun ibn Abi Shabib, from al-Mughirah ibn Shu'bah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever narrates a hadith while he considers it to be a lie, then he is one of the liars."