Who were the early scholars who considered Jihad the sixth pillar of Islam, and what were their proofs for that?

Scholars have agreed that Jihad is the best of voluntary acts after the obligatory duties, and that it is superior to Hajj, Umrah (definition), and voluntary prayers and fasting. Some have considered it a sixth pillar of Islam, while others believe it is not among the pillars, but rather the peak of Islam's hump (dhīrwat sanām al-Islām), because it is a collective obligation (fard kifayah) and may not continue until the end of time, unlike the five pillars. The disagreement over counting Jihad as a pillar is a verbal dispute that has no practical effect, as all scholars agree that Jihad is a collective obligation and may become an individual obligation (fard ayn) in certain circumstances.