def popular_trip(df):
    """
    Provides most popular trip. (i.e. Most commonly used start and end station combination.)
    :param df: DataFrame.
    :return: User message with popular trip stations to print.
    """
    trips_station = df.groupby(["Start Station", "End Station"])['Start Time'].count()
    sorted_trip_stations = trips_station.sort_values(ascending=False, axis=0)
    return "The most commonly used station combination is: \nStart station:{}\nEnd station:{}." \
        .format(sorted_trip_stations.index[0][0], sorted_trip_stations.index[0][1])