Question

What issue initially created a source of contention and division for Muslims into Sunni and Shi'a?

Answer 1

Answer:

The issue that was created a source of contention and division for Muslims is who should have the right to leadership.

What is the Sunni Shia split about?

Shia is known to have the opinion that Ali is to be the new leader as he was a relation of Muhammad.

The Sunni are known to want the best person for the job that is capable of leading  the community and as such, Sunnis view prevailed  and He was said to have became the 4th Caliph (which is leader).

Conclusively, The issue that was created a source of contention and division for Muslims is who should have the right to leadership as there was contention on who to rule.

thanks to user :

martebi

for the original answer :)