After gradual research; I have come to the conclusion that long before all heavenly books, God had revealed to the Hindus, through the Rishis of yore, of whom Brahma was the Chief, His four books of knowledge, the Rig Veda, the Yajur Veda, the Sama Veda and the Atharva Veda. The Quran itself made veiled references to the Upanishads as the first heavenly book and the fountainhead of the ocean of monotheism.