webpack.prod.config.js

...
var ExtractTextPlugin = require("extract-text-webpack-plugin");
...
module.exports = {
	module: {
		loaders: [
			{ test: /\.css$/,    loader: "style!css" },
			{ test: /\.scss$/, loader: ExtractTextPlugin.extract('style-loader', ['css-loader', 'postcss-loader', 'sass-loader'])},
			...
		]
	},
	plugins: [
		...
		
        new ExtractTextPlugin('styles/style.css')
	],

	...
};