In Alex's answer to Did any Achronim comment on the American Revolutionary or Civil Wars? he says that a certain Rav was opposed to the Emancipation Proclamation, because it goes against Noach's curse, that Cana'an and his descendants will be slaves.

I remember reading, perhaps in a Torah commentary, that when one of the Canaanite nations heard about Beni Yisrael coming to the Land, they were afraid, and went to Africa. This would make Africans, or at least certain tribes, descendants of Cana'an.

Does anyone know the source of this claim?