Gulp.js: Process SASS to CSS and auto-prefix the necessary properties

gulpfile.js

var gulp = require('gulp'),
	gutil = require('gulp-util'),
	sass = require('gulp-ruby-sass'),
	autoprefixer = require('gulp-autoprefixer');

gulp.task('css', function() {
	return gulp.src('./source/assets/css/main.scss')
		.pipe(sass({
			style: gutil.env.production ? 'compressed' : 'expanded'
		}))
		.pipe(autoprefixer({
			// autoprefixer config
		}))
		.pipe(gulp.dest('./build/assets/css'));
});

Any ideas how to create proper sourcemaps with the prefix-modified version?