Is Leaving Sunnah a Sin?
Question

All praise is due to Allah, we ask Him for His guidance and we seek refuge from the evil within ourselves.
As Allah said " Fas alu ahla ath-thikri in kuntum la ta'lamun". 
So my Question is:
1. Will Allah punish us if we leave the Sunnah of the Prophet (peace and blessings be upon him)?
Note: Any kind of Sunnah whether it's shaving men's beard or not praying 2 rak`ah before Fajr.
Praise be to Allah.What is Sunnah?
The word Sunnah has two meanings:

The path of guidance, which is the meaning in many hadiths, such as the words of the Prophet (peace and blessings of Allah be upon him): Whoever turns away from my Sunnah has nothing to do with me.
What the scholars of Usul and Fiqh call mustahabb (encouraged), which are deeds for which the one who does them will be rewarded, but the one who does not do them will not deserve to be punished for that. Examples include al-Sunan al-Rawatib (Sunnah prayers which the Prophet (peace and blessings of Allah be upon him) did regularly), Duha prayer and so on.

Which Sunnah will you be punished if you miss it?
Therefore, a person will not be punished for not doing things which are Sunnah in the second sense. With regard to the first definition, this is not the case, for these are divided into things which are obligatory (wajib) and things which are supererogatory (nafl).
Sunnat al-Fajr and al-Witr are Sunnah Muakkadah, prayer which the Prophet (peace and blessings of Allah be upon him) never omitted to do, whether he was travelling or not.
Letting the beard grow is an obligation (wajib); it does not come under the category of Sunnah as defined by the jurists. Whoever shaves his beard is imitating the Majus (Zoroastrians), going against the Fitrah (natural disposition of man), and changing the creation of Allah.
The one who neglects an act that is an obligatory Sunnah will be punished; the one who neglects a mustahabb Sunnah will not be punished, but he is missing out on a great reward and the chance to make up any shortfall in his obligations, because on the Day of Resurrection, any shortfall in his obligations will be made up from his Sunnah actions, if he has any to his credit. Doing Sunnah actions is also a means of maintaining the practice of obligatory actions.
The scholars also use the word Sunnah in contrast to bid’ah (reprehensible innovation) and say Ahl al-Sunnah (the people or followers of Sunnah) to distinguish them from the followers of misguided disbelieving sects such as the Jahmiyyah, or innovators who are not disbelievers, such as the Ash’aris and others. 
The Sunnah in this sense has to be followed . Following the way of Ahl al-Sunnah is obligatory, and whoever departs from their way is doomed.
Imam Malik (may Allah have mercy on him) said: “The Sunnah is like the ship of Nuh: whoever boards it will be saved and whoever stays behind will be drowned.”
And Allah knows best.