A farmer in Bangladesh was preparing his land for planting crops. As he dug through the soil the equipments kept getting stuck in one place. Out of curiosity he decided to check out what was in there. He dug the ground and discovered a big box. The farmer quickly opened the box to see what was in it but to his disappointment, there were only black stones inside the box. He decided to throw those stones at the birds when they come to eat the crops at the time for harvesting. Eventually, a day came when the crops were ready and the birds come to eat them. So, he started throwing the stones at the birds from where ever they showed up. One day a man was walking by the field who sells precious stones such as pearls, diamonds, rubies etc. Two stones happened to fall right in front of him when the farmer threw them at the birds. The man brought the stones to the farmer and asked if he was willing to sell them to him for $5,000 each. The farmer thought that the man was joking so he replied, "No." Then the man offered $50,000 for each stone and explained that they were precious jewels. Now the farmer started crying saying that he had only 1 or 2 stones left and the rest were thrown away.
This story is mentioned in the book "20 Reminders" by Muhammad Abu Yusuf.
In fact, every second of our time is very valuable. Today, we are just throwing them away here and there. But on the Day of Judgement, we will realize how valuable they were but it will be too late. Now is the chance to use our time properly.