Ask HN: In what video did PG say: spend 80% of time building and talking to users

In a YC video, PG said teams are advised to spend 80% of their time building and talking to users. Any idea what video it was in? Also, are there any good stories that demonstrate why this is good advice.

Background: working on an expert system, that has "startup advice". It is story based advice, advice that has a story to back it up and give it context.