Cannon states that he was told by John Taylor in Nauvoo about Joseph Smith's priesthood ban teachings.
George Q. Cannon, Journal, December 16, 1897, Church Historian's Press, accessed April 21, 2021
The question also came up whether a white man who was married to a woman having negro blood in her veins could receive the Priesthood. I explained what President Taylor had taught me when I was a boy in Nauvoo concerning this matter; he had received it from the Prophet Joseph, who said that a man bearing the Priesthood who should marry or associate with a negress, or one of that seed, if the penalty of the law were executed upon him, he and her and the offspring would be killed; that it was contrary to the law of God for men bearing the Priesthood to have association with that seed. In this case submitted to us a white man had married a woman with negro blood in her ignorantly; yet if he were to receive the Priesthood and still continue his association with his wife the offspring of the marriage might make a claim or claims that would interfere with the purposes of the Lord and His curse upon the seed of Cain.