Fast when you can see the crescent moon in the sky and stop fasting when you can see it again at night.

The Prophet Muhammad taught us that fasting should be done according to the sighting of the crescent moon. When we can see it in the sky, we should start our fast and when we can see it again at night, then our fast has ended. However, if due to clouds or other reasons we cannot see it in the sky, then we should count thirty days as a precautionary measure. This hadith teaches us that Muslims must follow certain rules while fasting such as counting days accurately so that their fasts are valid and accepted by Allah. It also teaches us about being mindful of our actions and following instructions from Allah's Messenger carefully.