The sunnah and avoid extremism.

 The sunnah and avoid extremism. 

The sunnah is to practice how the prophet muhammad salla Allahu alayhi wasalam practice islam. 

Going beyond it is extremism as reported in Hadith, 

Narrated Abu Huraira:

The Prophet (ﷺ) said, "Religion is very easy and whoever overburdens himself in his religion will not be able tocontinue in that way. So you should not be extremists, but try to be near to perfection and receive the good tidings that you will be rewarded; and gain strength by worshipping in the mornings, the afternoons, and during the last hours of the nights." (See Fath-ul-Bari, Page 102, Vol 1).. 

Another hadith that confirms not to do much than the sunnah is that of the three men who came to ask how the prophet peace be upon Him worship. 

Anas (Allah be pleased with him) reported that some of the Companions of Allah's Apostle (ﷺ) asked his (theProphet's) wives about the acts that he performed in private. Someone among them (among his Companions) said:

I will not marry women; someone among them said: I will not eat meat; and someone among them said: Iwill not lie down in bed. He (the Holy Prophet) praised Allah and glorified Him, and said: What has happened to these people that they say so and so, whereas I observe prayer and sleep too; I observe fast and suspend observing them; I marry women also? And he who turns away from my Sunnah, he has no relation with Me

Sahih Muslim 1401.

So it is not allowed to go contrary to the sunnah for that is not part of worship.

We practice islam as we saw or heard the prophet muhammad peace be upon him practicing it.