Allah the Exalted, through time, had sent several Messengers to mankind. Some of those Messengers were given books, which were Allah’s words and revelation to them and their people. Those Books contained guidance to mankind; guidance in belief, worship, rulings, and manners.
The Poofs for this:
Allah the Exalted said: (The Prophet believed in what was revealed to him and the believers did; All believed in Allah, His angels, His books and Messengers) (The Noble Qur’an 2:286)
The previously mentioned famous hadith of Jibreel (… Jibreel said: Tell me about Iman. The Prophet said: That you believe in Allah, His angels, His books, His Messengers, and the Last Day, and to believe in the Decree, the good of it and the bad of it. ….) (Muslim)