The Quran

وَكَذٰلِكَ أَوحَينا إِلَيكَ روحًا مِن أَمرِنا ۚ ما كُنتَ تَدري مَا الكِتابُ وَلَا الإيمانُ وَلٰكِن جَعَلناهُ نورًا نَهدي بِهِ مَن نَشاءُ مِن عِبادِنا ۚ وَإِنَّكَ لَتَهدي إِلىٰ صِراطٍ مُستَقيمٍ

Muhammad Asad

And thus, too,¹ [O Muhammad,] have We revealed unto thee a life-giving message,² [coming] at Our behest.[Ere this message came unto thee,] thou didst not know what revelation is, nor what faith [implies]:³ but [now] We have caused this [message] to be a light, whereby We guide whom We will of Our servants, and, verily, [on the strength thereof] thou, too, shalt guide [men] onto the straight way -

I.e., in all the three ways mentioned in the preceding verse.
The term ruh (lit,, "spirit" or "soul") has in the Qur'an often the meaning of "divine inspiration" (see surah 16, note 2). In the present context, it evidently denotes the contents of the divine inspiration bestowed on the Prophet Muhammad, i.e., the Qur'an (Tabari, Zamakhshari, Razi, Ibn Kathir), which is meant to lead man to a more intensive spiritual life: hence my above rendering.
I.e., that the very concept of "faith" implies man's complete self-surrender (islam) to God.