What Is The Mekor To Saying Hamokom Yinachem By A Shiva

The Minhag for Ashkenazim, when being Menachem Aveilim, is to say:

 “ Hamakom Yenachem Eschem B’toch Shaar Aveilei Tzion V’yerushalayim”

 Some Aveilim have this text hanging behind them, so that the Menachmim will know to say the exact Nusach. 
 This text does not exists in Bavli, Yerushalmi, Rambam, Tur or Shulchan Aruch. 

 The first source of this Nusach, is in Shu”t Shvus Yaakov (3 : 98) He writes if one of the Aveilim is Menachem other Aveilim he should say Hamakom Yenachem Eschem VEOSONU