Following the Sunnah is not "optional" if one wants to obey the Messenger of Allah.

Quran 24:52 Only those who obey Allah and His Rasool, have fear of Allah and do good deeds, are the ones who will be the winners.

Quran 3:131-132 Guard yourself against the Fire which is prepared for the unbelievers. Obey Allah and His Rasool so that you may find mercy.

Quran 4:64 We did not send any Rasool but to be obeyed by Allah's leave. If they would have come to you when they had wronged themselves to seek Allah's forgiveness and if the Rasool had also asked Allah's forgiveness for them, they would have found Allah Forgiving, Merciful.

Quran 4:80 Anyone who obeys the Rasool, in fact, obeys Allah. As for those who pay no heed, they should know that We have not sent you as a taskmaster over them.

Quran 47:33 O believers! Obey Allah and obey His Rasool, and do not let your deeds come to nothing.

Quran 8:20-21 O believers! Obey Allah and his Rasool and do not turn your back to him, now that you have heard all. Do not be like those who say: "We hear," but give no heed to what they hear.

Leaving the beard to grow is clearly an example of being obedient to Prophet Muhammad.

Al-Bukhari 7.781 Allah's Apostle said, "Cut the moustaches short and leave the beard (as it is)."

Al-Bukhari 7.780 Narrated Nafi: Ibn Umar said: The Prophet said, "Do the opposite of what the pagans do. Keep the beards and cut the moustaches short." Whenever Ibn 'Umar performed the Hajj or 'Umra, he used to hold his beard with his hand and cut whatever moustaches. Ibn Umar used to cut his moustache so short that the whiteness of his skin (above the upper lip) was visible, and he used to cut (the hair) between his moustaches and his beard.