The Quran

ثُمَّ قَفَّينا عَلىٰ آثارِهِم بِرُسُلِنا وَقَفَّينا بِعيسَى ابنِ مَريَمَ وَآتَيناهُ الإِنجيلَ وَجَعَلنا في قُلوبِ الَّذينَ اتَّبَعوهُ رَأفَةً وَرَحمَةً وَرَهبانِيَّةً ابتَدَعوها ما كَتَبناها عَلَيهِم إِلَّا ابتِغاءَ رِضوانِ اللَّهِ فَما رَعَوها حَقَّ رِعايَتِها ۖ فَآتَينَا الَّذينَ آمَنوا مِنهُم أَجرَهُم ۖ وَكَثيرٌ مِنهُم فاسِقونَ

Muhammad Asad

And thereupon We caused [other of] Our apostles to follow in their footsteps; and [in the course of time] We caused them to be followed by Jesus, the son of Mary, upon whom We bestowed the Gospel;¹ and in the hearts of those who [truly] followed him We engendered compassion and mercy. But as for monastic asceticism² - We did not enjoin it upon them: they invented it themselves out of a desire for God's goodly acceptance.³ But then, they did not [always] observe it as it ought to have been observed:⁴ and so We granted their recompense unto such of them as had [truly] attained to faith, whereas many of them became iniquitous.⁵

See surah 3, note 4.
The term rahbaniyyah combines the concepts of monastic life with an exaggerated asceticism, often amounting to a denial of any value in the life of this world - an attitude characteristic of early Christianity but disapproved of in Islam (cf. 2:143 - "We have willed you to be a community of the middle way" - and the corresponding note 118).
Or: "they invented it themselves, [for] We did not enjoin it upon them: [We enjoined upon them] only the seeking of God's goodly acceptance". Both these interpretations are equally legitimate, and are accepted as such by most of the classical commentators. The rendering adopted by me corresponds to the interpretation given by Sa'id ibn Jubayr and Qatadah (both of them cited by Tabari and Ibn Kathir).
I.e., not all of them observed it in the right spirit (Tabari, Zamakhshari, Iba Kathir), inasmuch as in the course of time many of them - or, rather, many of those who came after the early ascetics (Tabari) - corrupted their devotions by accepting the ideas of Trinity and of God's incarnation in Jesus, and by lapsing into empty formalism (Razi).
Sc., "and were deprived of Our grace".