About Thus have I heard

Statue of Kumārajīva in front of the Kizil Caves, Xinjiang, China
Thus have I heard is the common translation of the first line of the standard introduction of Buddhist discourses. This phrase serves to confirm that the discourse is coming from the Buddha himself, as a "seal of authenticity". Buddhist tradition maintains that the disciple Ānanda used the formula for the first time, as a form of personal testimony, but this is disputed by some scholars. It is also disputed how the phrase relates to the words that follow, and several theories have been developed with regard to how the text was originally intended to be read. The formula has also been used in later Mahāyāna and Vajrayāna discourses.
Folio from a manuscript of the Aṣṭasāhasrikā Prajñāpāramitā Sūtra depicting Shadakshari Lokesvara

Different types of Buddhist texts & literature

Buddhist texts are those religious texts which belong to the Buddhist tradition. The earliest Buddhist texts were not committed to writing until some centuries after the death of Gautama Buddha. The oldest surviving Buddhist manuscripts are the Gandhāran Buddhist texts, found in Afghanistan and written in Gāndhārī, they date from the first century BCE to the third century CE. The first Buddhist texts were initially passed on orally by Buddhist monastics, but were later .