Sahih Muslim , Book 1, Hadith 273 (USC-MSA) ⓘ
It is narrated on the authority of Anas that verily the Messenger of Allah said: "The Hour (Resurrection) will not occur until 'Allah, Allah' is not said on earth."
حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا عَفَّانُ، حَدَّثَنَا حَمَّادٌ، أَخْبَرَنَا ثَابِتٌ، عَنْ أَنَسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لاَ تَقُومُ السَّاعَةُ حَتَّى لاَ يُقَالَ فِي الأَرْضِ اللَّهُ اللَّهُ " .
also referenced under: Book 1, Hadith 282 (In-Book) & Hadith 148 (Reference)