What is the Punishment of not Fasting in Ramadan?

Abu Umamah al-Bahili, may Allah be pleased with him, narrated:

-I have heard Allah's Messenger, peace and blessings upon him, say:

“While I was sleeping, two men came and, holding both of my arms, they brought me to a mountain which was hard to climb. And they said:

“Climb this.”

When I said “I can't.” they replied:

“We will make it easy for you.”

Thereupon, I started to climb the mountain. When we reached the middle, suddenly harsh voices were heard.

When I asked: “What are those voices?” they replied:

“The clamor of the people of Hell.”

We started again. We suddenly saw a group of people whose cheek pouches were split, and out of these pouches flew blood; and their feet were bound!

I asked: “Who are they?”

They replied: “They are those who eat their fasts before due time (those who don't fast).” (at-Targhib wa’t-Tarhib, 2:453.)