It is reported that a group of heretics (People of Bid’ah) and their devout worship was mentioned to ʿAbd Al-Raḥmān b. Mahdī. He said, “Allāh only accepts what conforms to what has been commanded and what is in the Sunnah.” Then he recited, And a monasticism which they (Christians) innovated, We did not ordain it upon them [Al-Ḥadīd: 27]. He went on to say, “So Allāh did not accept this from them and reprimanded them for it.” He then said, “Stick to the way and the Sunnah.”

Abū Nu’aym, Ḥilyah Al-Awliyā' Vol.4 p44.