Whether we allow people to ignorantly break this rule, rather than have them do so knowingly: Shabbat 148b
Source for Beginning Oppression early: Yoma 81b
Source for Beginning the Prohibition against Work early: Rosh HaShanah 9a, Yoma 81b
Divine ex-communication for Violation of Beginning Oppression early: Yoma 81a
Divine ex-communication for Violation of Beginning the Prohibition against Work early: Yoma 81a
Source for adding time at the end: Rosh HaShanah 9a
Whether women are required in the additional time of "oppression": Succah 28b