During the first Buddhist council, both the dhamma and vinaya were recited so that the teachings can be compiled, consolidated and transmitted 3 months after Buddha's Mahaparinibbana. It lasted for 7 months.
Rajagaha was selected for the meeting of the council. It was held near the Saptapanni Cave. All other Bhikkhus not involved in the council were asked not to come to Rajagaha for the rainy retreat. It was sponsored by King Ajatasattu who was a firm believer in the Buddhist faith, and he would provide the necessary accommodation, food, and security for the gathering. It was chaired by Mahakassapa together with 499 arahants.
Why conduct the First Buddhist Council?
1.Delight and rejoice in the passing away of the Buddha
The old monk, Subhadda was happy that Buddha had passed away and they would no longer need to follow what to do and what not to do and that he could start doing whatever he felt like. He said “We have got rid of the Master.”
Mahakassapa found this news alarming as:
i. This unwholesome comment was made within the monk community.
ii. The body of the Buddha was still lying there and such a disrespectful comment could be passed.
iii. Mahakassapa had also witnessed the dispute and discord amongst the disciples of Mahaveero, the leader of Jains after he passed away before the Mahaparinibbana of the Buddha. He was afraid that if there is no rules, the monks will misbehave like them.
Mahakassapa was concerned that false doctrine, ie, Adhamma and Avinaya (no vinaya and no dhamma) would be established soon after the mahaparinibbana of the Buddha should there be confusion as to what Buddha had permitted and what was not. As Buddha mentioned that only vinaya and dhamma will be the guide for future sangha generation.
Read more: http://www.suttas.com/1st-buddhist-council.html