The Messenger of Allah ﷺ said, "Whoever narrates a hadith from me believing it to be false is himself among the worst of liars."
حدثنا عبد الله حدثني عثمان بن محمد بن أبي شيبة حدثنا ابن فضيل عن الأعمش عن الحكم عن عبد الرحمن بن أبي ليلى
قال رسول الله ﷺ من حدث عني حديثا يرى أنه كذب فهو أكذب الكاذبين
Its isnad is sahih. Uthman bin Muhammad bin Abu Shayba was trustworthy, honest, and reliable, he authored the Musnad and the tafsir, and he was a contemporary of Imam Ahmad. Ibn Fudayl is Muhammad bin Fudayl bin Ghazwan.The hadith was narrated by Ibn Majah 1:10 from Uthman bin Abu Shayba, and it was also narrated by Muslim 1:5 from the hadith of Samura and Mughira, and likewise Ibn Majah narrated it from the hadith of both of them, with their wording being "then he is one of the two liars." See also 584 and 630, and see also our commentary on Al-Risala of Shafii, 1098.This hadith is among the additions of Abdullah bin Ahmad in the K and H copies, but in the KH copy it is presented as part of the narration of Imam Ahmad himself, and our strong presumption is that it is indeed among Abdullah's additions.