what historical event was Lincoln talking about when he said?

“First, that no Negro slave, imported as such from Africa, and no descendant of such slave can ever be a citizen of any state in the sense of that term as used in the constitution of the United States. This point is made in order to deprive the Negro, in every possible event, of the benefit if that provision of the United States Constitution which declares that “the citizen of each state shall be entitled to all privileges and immunities of citizens in the several states”

The "House Divided" speech. ... About half-way through the speech.
ref: http://usinfo.org/enus/government/overview/22.html