Why was the school of thought of Imam Ahmad ibn Muhammad ibn Hanbal chosen for elaboration?

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The school of Imam Abu Abdullah Ahmad ibn Muhammad ibn Hanbal was selected because he possessed the greatest virtue, the closest means to God, was the most obedient to his Lord, and the most knowledgeable follower of the Messenger of God (peace be upon him). The rulings are dependent upon his opinions, and the jurists of Islam issue their fatwas according to his Madhhab (school of thought).