During the January 2020 Democratic debate at 43:27 the moderator asked Bernie why he said that a woman cannot win an election https://youtu.be/Ppm_mqo9Tyc?t=2607 . Bernie responds by denying that he ever said that. However, Warren confirms that he said it. Are there any earlier sources that confirm or deny whether Bernie actually said that?
Did Sanders say that a woman cannot be president or that that women in general lack the abilities?
Sanders responded that he did not believe a woman could win.
This is not a claim regarding women's inherent abilities, but a claim about sexism in the US which would prevent women from getting elected as president.
CNN gives as sources Warren who was present at the private meeting between her and Sanders, as well as four people who Warren spoke to after the meeting.
Sanders denies 1) that he said that women cannot be president (see video from OP) and 2) that he said that women cannot be elected president:
What I did say that night was that Donald Trump is a sexist, a racist and a liar who would weaponize whatever he could. Do I believe a woman can win in 2020? Of course! After all, Hillary Clinton beat Donald Trump by 3 million votes in 2016.
Conclusion: Warren doesn't claim Sanders said that women can't be president. Warren claims that Sanders said in a private meeting with her that a woman cannot win an election in the US in 2020. Sanders claims he only said that it would be more difficult for a woman to win an election in 2020. As this was said in a private meeting, we cannot know for sure which version is correct.
@tim's answer is excellent, but I'd like to address the question from a different angle.
This question's request for "earlier sources" is problematic. There were only two people present in the conversation, and they disagree. Any other claim of either verification or refutation is at best secondhand.
Since more direct evidence isn't available, it might be helpful to look for patterns of behavior or belief in earlier statements from Bernie Sanders. What has he said in the past about a woman as president?
- 1987: Bernie Sanders encourages girls to run for president
- 1988: "In my view, a woman could be elected president"
- 2015: "there is a desire on the part of many women, perfectly understandable, to see a woman being elected president. And we all want to see that."
- 2016: "Hillary Clinton will make an outstanding president"
Bernie Sanders is well known for the multi-decade consistency of his beliefs. (Refs: CNN, LATimes, NPR, NYTimes, DailyShow.) Sanders is also known for candor and authenticity, maintaining the same persona in public and private. (Refs: BostonGlobe, RollingStone, TheGuardian, OnTheCommons.)
None of this is direct evidence that he didn't say it, but it is strong evidence that the claimed statement would be highly out-of-character for him to say.