Answered by Sayyidi Habib Umar bin Hafiz (may Allah protect him and benefit us by him)
How do we respond to those that claim that Sayyiduna Muawiyah is not one of the Companions and that we should not say ‘may Allah be pleased with him’?
The best response is to continue to say ‘may Allah be pleased with him,’ and to avoid any argumentation. This is just one of a number of issues that certain people bring up today to cause friction, disunity, to waste Muslim resources, to stop them from having an influence on others and to show them as being divided. So we should focus on that which is of benefit and not allow them to achieve these base goals.