December 20. On this date in 1949, Shoghi Effendi wrote "The title 'Hidden Words' was given by Bahá'u'lláh because there is a tradition in Islam, that Fatimih, Muhammad's daughter, grieved greatly over the Martyrdom of her son, and God sent Gabriel to speak encouraging words to her. These words were 'hidden', only she having heard them, and in these latter days the Promised One has revealed them, thus fulfilling a tradition."

December 20. On this date in 1949, Shoghi Effendi wrote "The title 'Hidden Words' was given by Bahá'u'lláh because there is a tradition in Islam, that Fatimih, Muhammad's daughter, grieved greatly over the Martyrdom of her son, and God sent Gabriel to speak encouraging words to her. These words were 'hidden', only she having heard them, and in these latter days the Promised One has revealed them, thus fulfilling a tradition."

#1631. The Title 'Hidden Words' was Given by Bahá'u'lláh
"... The title 'Hidden Words' was given by Bahá'u'lláh because there is a tradition in Islam, that Fatimih, Muhammad's daughter, grieved greatly over the Martyrdom of her son, and God sent Gabriel to speak encouraging words to her. These words were 'hidden', only she having heard them, and in these latter days the Promised One) has revealed them, thus fulfilling a tradition."
(From a letter written on behalf of Shoghi Effendi to an individual believer, December 20, 1949)

Also here...

1631. The "Hidden Words"--A Collection of Gem-Like Utterances
"...the Hidden Words was originally designated the 'Hidden Book of Fatimih,' and was identified by its Author with the Book of that same name, believed by Shi'ah Islam to be in the possession of the promised Qa'im), and to consist of words of consolation addressed by the angel Gabriel, at God's command, to Fatimih, and dictated to the Imam Ali, for the sole purpose of comforting her in her hour of bitter anguish after the death of her illustrious Father. The significance of this dynamic spiritual leaven cast into the life of the world for the reorientation of the minds of men, the edification of their souls and the rectification of their conduct can best be judged by the description of its character given in the opening passage by its Author: 'This is that which hath descended from the Realm of Glory, uttered by the tongue of power and might, and revealed unto the Prophets of old....'"
(Shoghi Effendi: God Passes By, p. 140)

Please note that in the initial version above, Shoghi Effendi states "Fatimih, Muhammad's daughter, grieved greatly over the Martyrdom of her son," which contradicts the second version which states "for the sole purpose of comforting her in her hour of bitter anguish after the death of her illustrious Father." In fact, Fatimah died soon after her father and did not witness the martyrdom of either of her sons, Hasan or Husayn.