Is this Hadith
authentic?

Jazak'Allah.

RASULULLAH
(Sallallahu alayhi wasallam) said that The Hour of Qiyaamah will not come
except when the cross is worshipped inside the Mihraab of the Musaajid. 

Answer: 

Unfortunately we were unable to find an authentic narration regarding “the cross being worshipped in the Mihrabs of the Masaajid”.

However, the following quotations that are similar (albeit not exact) to what you have queried were found;

عن الحسن بن سمرة بن جندب قال لا تقوم الساعة حتي تروا امورا عظاما لم تكونوا ترونها ولا تحدثون بها انفسكم نا محمد ابن - وضاح قال انا اقول لاتقوم الساعة حتي تعبد الاصنام في المحاريب ابو بدر عن عبد الملك بن سعيد قال قال حذيفة رضي الله عنه لا تقوم الساعة حتي تنصب فيها الاوثان و تعبد )يعني في المحاريب حدثنيه ابن وضاح( } "البدع والنهي عنها" لابن وضاح صفحة - 91 دار الصفا {

Ibnu waddah makes mention that Qiyaamah will not occur until idols will be worshipped in the Mihrab.

Another narration on the authority of Huzaifah Radhiallahu Anhu states that Qiyaamah will not occur until idols will be placed and worshipped therein (in the Mihraabs)

It should be noted that these quotations do not link up to Rasulullah صلى الله عليه وسلم and there are academic technicalities found in the chain of narrators.

AND ALLAH TA'AALA ALONE IN HIS INFINITE KNOWLEDGE KNOWS BEST

ANSWERED BY: Ridwaan Hashim

CHECKED AND APPROVED BY:  Mufti Muhammed Saeed Motara Saheb D.B. 

Islamic Date:5 Jumaadal Ula1440  English Date: 12 January 2019