Hadrat  Imam-i Rabbani says in the 67th letter of the second volume of his Maktubat:
We should avoid the harams  as much as possible. We should deem the things which Allahu taala dislikes as fatal poisons. Thinking about our faults, we should feel ashamed, embarrassed for having done them. We should repent and be sorry. This is only becoming of His slaves. He who talks and acts in a manner which Allahu taala dislikes without being ashamed, embarrassed, has been persistently opposing Allahu taala. Their obstinacies will almost drive them out of Islam and will include them among His enemies. O our Allah! Have mercy on us. Through Thine Favour and Mercy, do not let us leave the right way.
It is written in the third paragraph of the second chapter of the fourth section of the book Riyad-un-nasikhin, Though the harams have been divided into two classes, grave sins, and small sins, we should avoid the small sins as well as we avoid the grave ones, nor should we slight any sin. For Allahu taala is muntaqim  and ghani (independent) . He does not fear anybody in doing what He wishes. He has concealed His wrath, His enmity in sins. Any sin which is supposed to be small may cause His vengeance, His wrath.