star wars - When & how did Yoda come to know about the Rule of Two?

At the very end of Star Wars Episode I: The Phantom Menace, Mace Windu and Yoda had this conversation:

Mace Windu: There is no doubt. The mysterious warrior was a Sith.
Yoda: Always two there are....no more...no less. A master and an apprentice.

When did he come to know about the secret Rule of Two? And, how?
Any evidence from Legends?

Answer

In the book Jedi vs Sith: The Essential Guide to the Force (pages 134-135), the story of Almas and a Jedi named Kibh Jeen is told. Jeen and his master had gone to the planet Almas in the Cularian system to investigate a "powerful infection of dark side surround the fortress on Almas."

The fortress was known to be "an ancient Sith Fortress" but "the Jedi and the Republic forgot about the Cularin system" until an explorer rediscovered it.

While there, Kibh Jeen killed his master and succumbed to the dark side. He later dominated the minds of some pirates and "unleashed his forces" on the third planet of the Cularin system. This was referred to as the "Dark Jedi Confilct."

After seven years a Jedi Knight and her Padwan organized the Cularin system into an armada and lured the pirates into a trap. Kibh Jean did not escape and

Before he died, he was heard spouting gibberish about there always being no more or less than two Sith.

The entry by Jedi Master Lanius Qel-Bertuk (who was headmaster of the Almas Jedi Academy later asked,

Were Kibh Jeen's dying words about the Sith a threat or a warning?

Also, he stated that

Although the answers may be found within that fortress, these are not answers we should pursue without extreme caution.

The Academy at Almas was destroyed during the Jedi Purge, so the confirmation of Jeen's statement about the Rule of Two may have been researched and confirmed. The entry by Master Qel-Bertuk was made in 53 B.B.Y. which would allow for enough time for the question to be answered.