“Whoever initiates a good practice (sunnah hasanah) in Islam and is emulated by others in doing so will get the reward of it and the reward of all those who act upon it without their rewards being diminished in any respect. And whosoever initiates an evil practice (sunnah sayyi’ah) in Islam and is emulated by others will bear the sin of it and the sin of all those who act on it without their burden being diminished in the least. ”
[Sahîh Muslim: (1017)]

Explanation:
The context of the hadith states that a group of poor people came to the Prophet (saw) so he asked those around him to give charity, but no one came forward - so much so that signs of anger could be discerned on the face of Prophet (saw), so one of the companions stepped forward and gave charity, so the Prophet (saw) said the above hadith.

Firstly, the word 'sunnah' which is used in this hadith cannot be understood to mean the Sunnah of Prophet (saw) because that would imply that there is something bad in the Sunnah; rather it is to be understood in its linguistic meaning of 'practice'.

This action the companion did was not something new in Islam, since giving charity was already legislated from the very first days of Isaam, rather he was simply implementing it, so the statement of the Prophet (saw) "a good sunnah" was said at a time when the people were reluctant to give charity, so one man started to give the charity and others followed him in it. Thus, he revived a Sunnah at a time when the people were reluctant to practice it, and this is the meaning of "a good sunnah." Hence, in the early works of 'aqeedah, this hadith was included under the chapter headings, "The reward of the one who renews the Sunnah." [Sharh Usool I'tiqaad 1/50]

The hadith uses the terms 'good' and 'bad', and the Shariah has already defined in its totality all that is good and all that is bad.

And the Ahlu Bi'dah use this hadith to justify their bid'ah - a feeble example.