Some time later, Paul said to Barnabas, "Let's go back and visit all the brothers and sisters in every city where we preached the Lord's word. Let's see how they are doing."
References for Acts 15:36
Barnabas wanted to take John Mark with them.
Paul insisted that they shouldn't take him along, since he had deserted them in Pamphylia and hadn't continued with them in their work.
Their argument became so intense that they went their separate ways. Barnabas took Mark and sailed to Cyprus.