The obligatory (fard) is:
That which is
(a) established by a decisive text (such as the Qur’an or hadiths transmitted by multiple contiguous chains (mutawatir)), and
(b) is decisive in what it indicates (i.e. is not open to multiple interpretations, etc.)
The necessary (wajib) is:
That which does not have one of the above, but indicates something mandatory
For example, something established by a probabilistic text (such as a non-mutawatir hadith) whose indication is decisive. [They mention that the fard is generally established by the Qur'an, and the wajib is generally established by the sunna, because all of the Qur'an is decisively established, while most of the Sunna is probabalistic.]
The confirmed sunna (sunna mu’akkada) is:
Something the Prophet (Allah bless him and give him peace) did without fail, and is not from is worldly habits.
Legal Consequences:
1. Leaving a fard and wajib is sinful, even if only once.
2. If a fard within an action is left out, the action is invalid.
3. If a wajib is left out, the action is valid but grossly deficient. As a result:
(a) it is wajib to repeat the action within the time to lift the sin.
(b) After the time goes out, it is recommended, though not wajib, to repeat the action. [In the prayer, if a wajib was forgetfully left out, for which one may perform forget prostrations]
4. Leaving a confirmed sunna (sunna mu’akkada) once without excuse is worthy of reproach and censure (from Allah), but it is not quite sinful.
5. However, leaving a confirmed sunna repeatedly or making this a habit is sinful, but less sinful than leaving a fard or wajib, for the Prophet of Allah (blessings and peace be upon him) said, “Whoever turns away from my sunna is not of me.” (That is, not of those who follow him as Allah has commanded to them.)
[Note: this opinion, of it being merely recommended to repeat a prayer in which a prohibitively disliked (makruh tahriman) act was performed or a necessary (wajib) act left out after the time has gone out is the dispensation, and was mentioned by Ibn Nujaym in his Bahr, al-Haskafi in his Durr, and al-Tahtawi in his Hashiya `ala al-Maraqi; Ibn Abidin, however, dismissed this opinion, arguing strongly that it remains wajib to make it up after the time. The former opinion is easier for those who would otherwise have a huge number of makeup prayers. For more recent blunders that one is sure of, and is free of doubts and waswasa, the latter opinion should be adopted. Those who have waswasa should stick to the former opinion.]
Wassalam,
Faraz Rabbani
