The Messenger of Allah (sal Allahu alaihi wa sallam) said:
“Surely, he who lives from among you will see much differing, so it is upon you to be upon my Sunnah and the Sunnah of the Rightly Guided Caliphs. Bite on to it with your back molar teeth and beware of newly invented matters, for verily, every newly invented matter is an innovation, and all innovation is misguidance.”
Amongst the most dangerous innovated matters are innovations in belief. The Messenger of Allah’s advice to us is to hold fast to the beliefs that he (sal Allahu alaihi wa sallam) expounded and that which the Khulafa Rashideen - Abu Bakr, Umar, Uthman and Ali (radi Allahu anhum ajmaeen) - held.
The scholar of the generation of the Taba-Tabaee, Abdullah Ibn al-Mubarak, lived to see the development of various sects and subsequently warned people from taking knowledge from them. Among the heretical views that Abdullah Ibn al-Mubarak opposed was the claim of the Murjiah that Eeman does not increase or decrease, the claim of the Jahmiyyah and Qadariyya that the Quran is created, and the Mutazili denial of core Sunni beliefs such as the punishment of the grave.
As a Muhaddith and Faqeeh, he made public his abandonment of those who propagated deviancy. He held sincere to his beliefs and strived to follow the Sunnah as best as he could. As a result, his efforts against anything which misguided people away from the Sunnah of Muhammad (sal Allahu alaihi wa sallam) were strict and firm.
It once reached Abdullah Ibn al-Mubarak that al-Harith al-Muhasabi (author of Risalat al-Mustarshidin) ate some food with a known innovator, so Ibn al-Mubarak said to him, “I am not going to speak to you for thirty days!” [Hilyatul Awliya]
Abdullah Ibn al-Mubarak’s advised: “You should know my brother, that death is a blessing for every Muslim who meets Allah upon the Sunnah. Indeed, to Allah we belong and to Him we are returning! Only to Allah do we complain of our loneliness for indeed our brothers have gone, our supporters have decreased and innovation has appeared.” [Al-I'tisam, by Imam al-Shatibi].