Bava Batra 30 - Presumptive Ownership (Chazakah) Case Law #1
One man told the other, "What are you doing in this house?" The occupant replied, "I bought it from you, and I have used it for three years needed to establish chazakah." The original owner replied, "I was residing in the inner rooms all this time."
The litigants came before Rav Nachman, who said to the occupant, "Prove your exclusive use of the house by producing witnesses that you lived there alone, and you will have established chazakah."
Rava said to Rav Nachman, "Is that the law? The burden of proof rests upon the one who seeks to exact payment from his fellow, that is, on the previous owner!" - but the law is like Rav Nachman, since his view always prevails over Rava's.