Question Summary:
Denying a Sunnah practice

Question Detail:

 
It is mentioned in many books of fiqh that saying "No" to someone who asks you to do a sunnah leads to kufr. For example it is mentioned in many books of fiqh that somebody who tells you "Cut your nails, it's a sunnah" and you say "no" then you become a kafir.
But this seems like it must relate to intention, because I was on a picnic recently and I was doing a ghusl because it was Friday and I told my friend to do one too, and he said No. and I knew he meant because time was tight for prayer. 
Likewise sometimes people ask someone to do something like "Fix such and such a problem" and they reply "inshaa Allah" and it is clear they don't want to do it and one responds "No" i.e. no don't be lazy not no to the will of Allah.
Could you please clarify this issue, and why the fuqaha mention it in their books?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
If a person refuses to carry out a Sunnah due to looking down upon the Sunnah, then only it will lead to kufr[1]. If he refuses to carry out the Sunnah due to constraints etc, that will not be kufr[2].
 
And Allah Ta’āla Knows Best
Immad Bin Arshad
Student Darul Iftaa
California, USA
Checked and Approved by,
Mufti Ebrahim Desai.
www.daruliftaa.net