She further mentioned that it appears in many sources including Dale Kent's book on Cosimo de Medici. I was unable to track down that source on the internet, but found the Poliziano passage in a work by Douglas Kries, Piety and Humanity, where he cites Weissman below.
I invite you to cry with Him in His bitter pain, . . . to become His disconsolate widow; [Emphasis added.] to see His grieving mother whose heart was pierced with a knife; to cry together with the stones, the sun, with heaven and earth, with all the elements, with the whole world over His incomparable torment. . . . Let us cry tenderly at the death of sweet Jesus. . . . Let us do penance, and with devout contrition let us humiliate ourselves before God.Interestingly McGowan points out that no one questions the reference to "His disconsolate widow." Kries (1997: 36) is no exception. He discusses the "provocative" imagery of "His grieving mother whose heart was pierced with a knife," but ends up describing the passage as Poliziano urging his listeners to "engage in flagellation."
SOURCE: Ronald Weissman, "Sacred Eloquence: Humanist Preaching and Lay Piety in Renaissance Florence," Christianity and the Renaissance (Syracuse, NY: Syracuse University Press, 1990), p. 261.