"What is the fitna (trial/sedition) of Abdullah ibn Saba' al-Yahudi (the Jew), which led to the increased questioning and investigation of isnad (chain of narration)? "

Praise be to Allah, and blessings and peace be upon the Messenger of Allah and upon his family and companions. At the beginning, Muslims did not ask about the isnad (chain of narrators), but after the turmoil (murder of Uthman and the emergence of the Khawarij), they began to seek knowledge of the narrators of the Hadiths. So, they would accept the Hadiths of the people of Sunnah and reject the Hadiths of the people of innovation. Scrutiny in isnad increased especially after the turmoil of Abdullah Ibn Saba, the Jew who spread innovations and misguidance. Scholars from the companions and followers would not accept Hadiths except after knowing their narrators and ensuring their trustworthiness and integrity, in order to preserve the prophetic Sunnah from fabrication and distortion.