In the sefer HaYom Yom, this entry is found and attributed to the 5th Lubavitcher Rebbe, the entry for the 3rd of Av...

When Mashiach will come (speedily in our time, Amein), then we shall really long for the days of the exile. Then we will truly feel distress at our having neglected working at Avoda; then will we indeed feel the deep pain caused by our lack of Avoda.

These days of exile are the days of Avoda, to prepare ourselves for the coming of Mashiach, speedily in our time, Amein.