The phrase "Thus have I heard" (Pāli: Evaṃ me sutaṃ) is the standard opening for almost all Buddhist discourses. It serves as a seal of authenticity and dates back to the First Buddhist Council, held shortly after the Buddha's death.
Here is the story behind it:
The Speaker (Ananda): The "I" refers to Venerable Ananda, the Buddha's cousin and personal attendant for 25 years. He had a photographic memory and was present for almost all of the Buddha's sermons.
The First Council: After the Buddha passed away, 500 enlightened monks gathered to compile the teachings to prevent them from being lost or corrupted. Ananda was called upon to recite the discourses (Suttas) from memory.
The Meaning: By starting with "Thus have I heard," Ananda was making a crucial distinction:
"I am not the author of this."
"I am not interpreting this."
"I am merely reporting exactly what I heard the Buddha say."
It was a way to guarantee to the assembly that the teaching was authentic and preserved exactly as it was spoken. It marks the beginning of the oral tradition that passed the Dhamma down for centuries before it was written down.