The following citation is taken from William Cole’s article, “Was Luther a Devotee of Mary?,” found in Marian Studies, Volume XXI, 1970, p. 131:

In a Christmas sermon of 1531, Luther speaks of Mary as the “HIGHEST WOMAN AND THE NOBLEST GEM in Christianity after Christ.” He goes on to claim that “she is nobility, wisdom and holiness PERSONIFIED. WE CAN NEVER HONOR HER ENOUGH. Still honor and praise must be given to her in such a way as to injure neither Christ nor the Scriptures” (WA 34, 2, 497 and 499). 
(Capital emphasis mine)

Here are the links to the primary sources cited by William Cole (WA 34, 2, 497 and 499):

WA 34, 2, 497

WA 34, 2, 499

I want to thank a specific rabid anti-Catholic polemicist (whose name and post will not be shared here) for basically affirming the veracity of these quotes.

Ironically, while acknowledging the authenticity of these statements he still goes on a rant in his desperate attempt of explaining why such praise and honor of the holy Theotokos.