I found this excerpt on a web site,

http://sunsite.berkeley.edu/Literature/Douglass/Autobiography/05.html

but canot as yet locate Frederick Douglass's writings in which the quote appears.

"Despite this there are a number of primary sources for learning more about the lives of African-Americans who lived under slavery. Amongst the most famous first hand accounts of slave life in America are the writings of Frederick Douglass, in particular his 1845 publication the Narrative of the Life of Frederick Douglass, an American Slave, Written by Himself, which is available in digital form from the EDSITEment reviewed web resource The American Memory Project. Douglass’ account describes in detail what the life of a slave in early nineteenth century America consisted of- descriptions which highlight the brutality and the humiliation inherent in the practice. His writings also provide a moving and eloquent argument for an end to the South’s “peculiar institution” in the land of liberty. "