This question is asked in Al Hussain Foundation web site ( Here
and Answer by Maulana Meheboob Razvi (Ihya Academy)
 

Question: Assalamu Aalaikkum Hazrat..
Please Clarify the difference between, Farz, Sunnah, Vajib, Nafhil….

Answer:
Wa alaikumussalaam va rahmatullahi vabarakathuhu, Aap ka sawaal mousool huwa Jawaab hazir hai:

Istelahat-e-Shariah me:

Farz: Woh kaam hai ke Jaan bujkar chodna sakt gunah aur jis Ibaadat ke andar wo ho bagair is ke Ibaadat durst nahi.

Wajib: Woh kaam hai ke us ko jaan buj kar chodna gunah aur namaaz me qasdan chodne se namaaz ka dubara padhna zaroori aur bhul kar chot jaye to Sajdah sahu lazim.

Sunnat: Woh Mubarak fael, qoul o takreer hai jo ke mere aqa huzoor sallallahu alaihivasallam se sabith ho, Sunnath ke do qism hai:
Sunnat-e-muakkida: Jis ka karna sawaab aur chodna bura aur karna sawaab aur ittefaqan chodne par Etaab (kehr) aur chodne ki adat karne par mustahak-e-azaab.
Sunnat-e-gair muakkida: Jis ka karna sawaab aur chodna agarchen adatan ho Itaab nahi magar shariat ko pasand nahi.

Nafil: Jis ka Sawaab aur Na karna gunah nahi, magar Jis ka karna shuroo kiya aur kisi bhi wajay se toddiya ab uspar woh wajib hai ke usko pura kare.

(Anwar-e-Shaiat).
Allahu alam va Rasoolullah alam azzawajalla va sallallahu alaihiwasalam.
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