In another passage Ibn Taymiyyah—may Allah have mercy on him—spelled out why a weak ḥadith is reinforced by additional chains, the conditions for such reinforcement, and the necessity of adhering to this rule. He states in Majmūʿ al-Fatāwā, 13/347:

“When the marāsil (reports whose chains stop with a Successor) are transmitted through several independent routes and these routes are free from intentional collusion—or from any unintentional but coincidental agreement—they are unquestionably sound. A report is either true, accurately reflecting what happened, or it is false because the transmitter deliberately lied or erred. Whenever both deliberate lying and error are ruled out, the report is, without doubt, true.

If a ḥadith reaches us through two or more separate avenues—such as the ḥadith we are discussing—and we know that the informants did not collaborate in fabricating it, and we also know that the kind of detailed agreement found between them could not ordinarily occur by chance, we are certain of its soundness. It is like two people recounting an event they witnessed: each relates the same particulars of what was said and done. When we know that the second narrator did not coordinate with the first, we can be sure that the incident took place in general, for if each of them had fabricated or mistaken it, it would be virtually impossible for both to give those very same details without prior collusion.

Through this method we ascertain the truth of most reports whose diverse chains multiply in this fashion, even though no single chain may suffice on its own, whether because it is mursal or because its transmitter is weak.

This foundational principle must be understood, for it is highly beneficial in reaching firm conviction about many transmitted materials—whether in ḥadith, Qurʾānic commentary, accounts of the Prophet’s campaigns, or the recorded statements and deeds of people.”}