Musannaf Ibn Abi Shayba 33:1070

Yazid ibn Harun narrated to us, he said: Juwaybir informed us, from al-Dahhak, who said: Umar ibn al-Khattab wrote to Abu Musa: "To proceed: Indeed, strength in work is that you do not delay the work of today for tomorrow. For if you do that, the tasks will accumulate upon you, and you will not know which of them to take up, and thus you will lose them. So, if you are given a choice between two matters, one of them for the worldly life and the other for the Hereafter, then choose the matter of the Hereafter over the matter of the worldly life, for the worldly life perishes and the Hereafter remains. Be in a state of fear of Allah, and learn the Book of Allah, for it is the springs of knowledge and the spring of the hearts."