Flowers for Shabbos

We know that people buy flowers for Shabbos and Yom Tov. Where can we find a source for this?

This custom is brought in the Midrash which says just as a rose is designated for Shabbos and Yom Tov (מה שושנה זו מתוקנת לשבתות וימים טובים) so are the Jewish people designated for the redemption of the future.