Are the narrations from the Salaf mentioned in Fiqh books like Al-Majmu' and Al-Mughni authentic and considered primary references for the opinions of the Salaf, even if their chain of transmission is not mentioned, or is it obligatory to search for them in other books to verify the authenticity of their attribution?

Al-Majmu’ and Al-Mughni are two great books of Fiqh (Islamic jurisprudence), and they are not references for authenticating mawquf (وقف) and maqtu’ (مقطوع) narrations. Most of the narrations found within them have connected chains of transmission (isnad). Each of the authors (of these two books) presents his methodology in conveying schools of thought (madhahib), opinions, and using narrations as evidence. There is no harm in sufficing with what they mentioned, and whoever desires more should search for the chains of transmission of these narrations in their primary sources. Kanz al-Ummal does not mention the chains of transmission; rather, it suffices by attributing the narrations. As for Musannaf Ibn Abi Shaybah, Musannaf Abd al-Razzaq, Al-Sunan al-Kubra by Al-Bayhaqi, Al-Tamhid by Ibn Abd al-Barr, and Al-Muhalla by Ibn Hazm, these are primary sources for chains of transmission. The mere existence of a narration with a connected chain of transmission is not sufficient to judge its authenticity.