A person in the Mosque asked Imam Ali (AS):

O’ Ali (AS), tell me, what is Wajib? And what is more Wajib? What is near? What is nearer? What is strange? What is stranger? What is difficult? What is more difficult? Imam Ali (AS) replied: Wajib is avoiding sin, and more Wajib is to repent (Tawbah) from sin. Near is Qiyamah (Day of Judgment), and nearer to that is death. Strange is disloyalty of this world, and stranger is attachment to this perishable world. The difficulty is going towards the grave, and even more difficult is to go to grave empty-handed [1].

[1] Osul Kafi, Vol. 1, Page 250 & Tahdhibul Kamal, Vol. 5, Page 79.

Source: en.shafaqna.com