What is the best practice for using both icon and label in a button should it be before the label or after ?
According to the article Where to Place Icons Next to Button Labels, from UX movement, the icon should come first:
Eyetracking research has shown that users scan from the left in a vertical movement. In order for icons to serve as a visual scanning aid, users need to see them before they see the button label. Placing them to the left of your button label allows users to see the icon first. If you place icons to the right of your button label, they’re not helping users scan because users see them last. If users read the label first, your icon serves no purpose other than decoration. Icons describe button labels, not the other way around.
Aline has a great link to research in his answer. I'll add that if the icon is on the left, you leave the right side available to add a downward pointing arrow to indicate a dropdown menu. Additionally, after some digging, I found way more examples of icon on the left than on the right: