Short answer
Sunnah mu'akkadah (confirmed sunnah) is a sunnah the Prophet ﷺ kept up regularly, such as the two rakats before Fajr. It is not obligatory, so missing it now and then is not a sin. But in the Hanafi school making a habit of leaving it without an excuse is blameworthy, and the Fajr sunnah is the most stressed of all.
Sunnah mu'akkadah is what the Prophet ﷺ did consistently and rarely left. Leaving it once is a loss, not a sin; leaving it habitually without excuse is wrongdoing (isa'ah) that deserves blame, close to leaving a wajib. Sunnah ghayr mu'akkadah, such as four rakats before Asr, may be left without blame.
Other madhhabs
- Shafi'i
- The regular sunnahs are not obligatory and leaving them is not a sin, though leaving them persistently is looked down on.
- Hanbali
- The regular sunnahs are not obligatory and leaving them is not a sin, though persistently leaving them reflects badly on a person.
Explanation
Sunnah prayers come in two levels. Sunnah mu'akkadah (confirmed) are the ones the Prophet ﷺ prayed almost without fail. Sunnah ghayr mu'akkadah (non-confirmed) are ones he prayed sometimes. In the Hanafi school the confirmed ones are the 12 daily rakats (2 before Fajr, 4 before and 2 after Dhuhr, 2 after Maghrib, 2 after Isha), the 4 before and 4 after Jumuah, and taraweeh in Ramadan.
None of these is fard, and none is wajib like witr. But Hanafi scholars do not treat the confirmed sunnah as optional extras. Leaving them as a habit, without a reason, is called isa'ah (wrongdoing): it is blameworthy and a person is answerable for it, though less than for a wajib. The two rakats before Fajr are the strongest; Aisha said the Prophet ﷺ was more careful about them than any other voluntary prayer.
- Valid reasons to leave them: real shortage of time, illness, or being on the move during travel. Keep the Fajr sunnah even when travelling.
- Missed one today: no sin; just do not make it a pattern.
- Reward: whoever keeps the 12 rakats has a house built for him in Paradise.
The full list is in what are the sunnah prayers. Whether missed sunnahs are made up is answered in do you make up missed sunnah prayers.
The evidence
Sahih al-Bukhari 1163
Sahih... لم يكن النبي على شيء من النوافل أشد منه تعاهدا على ركعتي الفجر ...
"The Prophet was not more regular in any voluntary prayer than in the two rakats of Fajr."
The Fajr sunnah is the most emphasised sunnah.
Sahih Muslim 728
Sahih... من صلى اثنتى عشرة ركعة في يوم وليلة بني له بهن بيت في الجنة ...
"Whoever prays twelve rakats in a day and night, a house is built for him in Paradise."
The reward for keeping the confirmed sunnahs.
Jami' at-Tirmidhi 413
Grading to be checked... إن أول ما يحاسب به العبد يوم القيامة من عمله صلاته ...
"The first of a servant's deeds he will be called to account for on the Day of Resurrection is his prayer."
Sunnah prayers make up for shortfalls in the fard.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, chapter on witr and nawafil. Leaving a confirmed sunnah habitually is isa'ah and deserves blame.
- Nur al-Idah with Maraqi al-Falah, Chapter on nawafil. Lists the confirmed and non-confirmed sunnahs of each prayer.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.