alt Hacker NewsMusk was told by Demis Hassabis in 2012, see https://time.com/6310076/elon-musk-ai-walter-isaacson-biogra...
Altman read Nick Bostrom's book Superintelligence, he mentions it in this post https://blog.samaltman.com/machine-intelligence-part-1
I also read that book in 2014, and it is how I came to understand this problem early.
2012 - 2015 seems to have been around the time a lot of people first started taking ASI and ASI existential risk very seriously as things that will be relevant in our lifetime, and when they decided to try to build and/or prevent it.