Bism Illah wa as salaamu alaykum.
These are some key ayaat from the Qur'an and ahadith that clarify the critical importance of following the Sunnah. It is only in accordance with the Qur'an and the Sunnah that a Muslim will have his deeds accepted.
Allah the Almighty has said in the Qur'an:
It is not for a believer, man or woman, when Allah and His Messenger have decreed a matter, that they should have any option in their decision. (Surah 33, Ayah 36)
Allah also says:
Whatsoever the Messenger gives you, take it; and whatsoever he forbids you, abstain from it. (Surah 59: Ayah 7)
The obligation to follow the Sunnah is from Allah, subhaana wa taala.
Furthermore, Allah's Messenger, salla Allahu alayhi wa sallam, said:
"None of you truly believes until his desires are subservient to what I have brought." (An-Nawawi - Forty Hadith)
"Whoever performs a deed that is not in accord with our affair shall have it rejected." (Al-Bukhari and Muslim)
The Prophet (salla Allahu alayhi wa sallam) used to teach the Sunnah to his companions in word and deed, and urged them to follow it, as he said in his hadeeth: “Adhere to my Sunnah” and “Whoever neglects my Sunnah does not belong to me.” ‘Abd-Allaah ibn Mas’ood (may Allaah be pleased with him) narrated that the Messenger of Allaah (salla Allahu alayhi wa sallam) said: “There is no Prophet whom Allaah sent to any nation before me, but he had disciples from among his nation, and companions who followed his Sunnah and obeyed his commands.” (Narrated by Muslim, no. 71).
Please don't take the Sunnah as a side dish in your life or an accessory in your wardrobe. It is something that we should learn and live, bi ithn Illah taala.