אבל שלשה שאכלו על שלחן אחד ואמרו עליו דברי תורה כאלו אכלו משלחנו של מקום (אבות ג:ג)

Reb Duvid Moshe of Chortkov zt’’l used to frequently fast. However, you couldn’t tell when he was fasting. His family members used to worry as he was weak from all his fasting.  His rebbetzin finally figured out a tactic to know if the Rebbe was fasting 

She surmised that the Rebbe sent his food to someone if the plate was empty. And the same with a glass of tea. If it was empty, then the Rebbe must’ve spilled it.  Because if he ate or drank, he would’ve had some leftovers.

One day, the rebbetzin served the Rebbe a glass of tea, and when she came to retrieve it, she saw it was empty, meaning that the tzaddik was fasting.  The Rebbetzin asked her husband: “You’re so weak. What do you think Hashem prefers – your fasting or bringing a korban? (When one eats and benches afterward, it’s considered bringing a korban to Hashem .)

The Rebbe thought a minute and then asked his rebbetzin to prepare him a meal.