This is the famous story of Upali's conversion to
Buddhism.
Upali was one of the chief followers of the Jain
master, Mahavira. Because of his intelligence, Upali
often appeared in public debates on behalf of the
Jains.
There was one incident where Upali had a debate with
the Buddha. At the end of the debate,Upali was so
impressed with the Buddha's teachings that he asked to
be the Enlightened One's follower. "Venerable Sir,
please allow me to be your follower".
To that, the Buddha answered, "Upali, you are at the
height of your emotions. Go home and reconsider it
carefully before you ask me again".
Upali was extremely impressed, "If it was any other
guru, he will parade a banner saying,'Mahavira's chief
lay-disciple has become my follower'. But you,
Venerable Sir, you asked me to go home and reconsider.
Now, I want to be your follower even more. I will not
stand up until you accept me".
Finally, the Buddha agreed to accept Upali, under one
condition, "Upali, as a Jain, you have always given
alms to Jain monks. When you become my follower, you
will CONTINUE to give alms to Jain monks. This is my
condition".
Upali agreed to this condition. He later became one of
the Buddha's chief disciples. Upali was known to be
the one who compiled the Vinaya, the rules for monks.
Vinaya: The monastic discipline, whose rules and
traditions comprise six volumes in printed text. The
Buddha's own name for the religion he founded was
"this dhamma-vinaya" -- this doctrine and discipline.
The essence of the rules for monastics is found in the
Patimokkha.