Sufyaan Ath-Thawree said:

“A person who sits with a heretic (an adherent of bid’ah) will not escape one of three things: (a) either he will become a trial (fitnah) for others, (b) or some deviation will occur in his heart and he will slip and be cast into the Fire by Allah, (c) or he will say to himself, ‘By Allah, I don’t care what they say, I am confident about myself,’ but whoever feels secure from Allah about his religion even for the blinking of an eye, Allah will take his religion away from him.”

[Ibn Waddâh, Al-Bida’ p125]

Imaam Ahmad records that ‘Ubaadah Ibn As-Saamit said to one of his colleagues:

“It is well possible that if you live a long life, you will see a man who read the Quraan from the tongue of Muhammad (saw), or from one who read it from Muhammad, repeating it and reciting it, treating its lawful as lawful, and its prohibited as prohibited. He will pass you on the way to his house, and his status in your eyes will be as insignificant as the head of a dead donkey.”

[Ahmad #17140 with a da’eef isnaad]