Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Dhahhak. He says that Hazrat Umar bin Khattab, may Allah be pleased with him, wrote to Hazrat Abu Musa: "Amma ba'd! Surely, there is strength in action. Do not leave today's work for tomorrow. Because if you do so, many tasks will accumulate upon you. You will not know which of them to take up. Then you will waste them. So when you are given a choice between two tasks, one of which is for the world and the other for the Hereafter, then give preference to the work of the Hereafter over the work of the world. Because the world is perishable and the Hereafter is everlasting. And remain in fear of Allah. And learn the Book of Allah, for it is the source of knowledge and the springtime of hearts."