It was reported live on the day by Sky News, first the Chief Whip went in to tell him in a VoNC only 65 MPs were likely to support him,, then about an hour latter they reported a resignation letter was being drawn-up, and after that, that he had signed it. He did his speech over 2 hours later, so plenty of time for careful wording that Johnson with agree too,
Impossible to find a source online now, as any search including 'Johnson' and 'resignation letter' returns results including all those 62 MPs that resigned, on a google news search almost 1.5 million results!