How sound is Imam Ahmad's statement, "Whoever claims there is a consensus is a liar," which is mentioned in Al-Muhalla by Ibn Hazm? And if it is sound, how can consensus be a source of legislation when it has not been formed? And has consensus ever been formed at any time, and are there books that compile these consensuses?

Imam Ahmad's statement, "Whoever claims consensus is a liar," does not negate the authoritativeness of scholarly consensus (Ijma'). Rather, it indicates the difficulty of verifying the agreement of all scholars and his rejection of claiming consensus to contradict textual evidence, as clarified by Ibn al-Qayyim. Indeed, consensus was established in the era of the Companions and the early generations on some issues, and there are books written on it, such as "Al-Ijma'" by Ibn al-Mundhir and "Maratib al-Ijma'" by Ibn Hazm.