System of Islam by Taqiuddin an-Nabhani

Linguistically Sunnah means the method. However, in terms of the Shari’ah it designates the nafilah that has been narrated from the Prophet (saw) such as the recommended rakaat (rakaat as-Sunnah) which are distinct from fard (compulsory). It should not be understood that the action is called Sunnah because it is from the Prophet and that the fard is from Allah (swt). The Sunnah and the fard are both from Allah (swt); and Rasool Allah (saw) is but a conveyor from Allah (swt), because the Rasool Allah (saw) uttered not out of whims but only that which was revealed to him from Allah (swt) i.e. wahy. Thus, although Sunnah is narrated from the Prophet (saw), nevertheless it is narrated as a recommended action i.e. nafilah that is why it is called sunnah; in the same way that the fard has been narrated as a compulsory action. Hence, the two compulsory raka'at of the dawn (fajr) prayer have been narrated from the Prophet (saw) through decisive reports, known as tawatur, as being fard; and the two recommended raka'at of the fajr prayer have also been narrated through decisive reports (tawatur) as being Sunnah (nafilah); and both are from Allah (swt) and not from Rasool Allah (saw) himself. Thus the command (amr) is either fard or nafilah in actions of worship (‘ibadat), and fard , mandoob or mubaH in other actions. In other words, nafilah is the same as mandoob, but it is called nafilah, and called as Sunnah.

The Sunnah also means all the shar’ai evidences which came from Rasool Allah (saw) other than the Qur'an. This includes his speech, actions and consent (his silence upon actions performed before him).