‘Abdullah ibn al-Mubaarak 181 AH (may Allah have mercy on him) said:

الاسناد عندي من الدين, لو لا الاسناد لقال من شاء ما شاء

“The chains of narrations (isnad) is from the religion; were it not for the isnad anyone could say anything they wanted.”

Taken from the introduction of Imaam Muslim’s Saheeh, and also found in Imaam Nawawi’s explanation of Saheeh Muslim 1/12