Our Prophet (saas) foretold a muslim community that will capture Istanbul with no bloodshed, merely by remembering Allah and His Oneness

Rasulullah (saas) one day asked his people:

I WILL DESCRIBE A CITY TO YOU. ITS POSITION IS LIKE THIS: ONE SIDE IS OPEN TO THE SEA, WHILE THE OTHER IS ON LAND. HAS ANYONE SEEN OR HEARD OF OR KNOW OF SUCH A CITY?”

They said:

We have heard of such a place. THAT PLACE MUST BE VERY WELL-MAINTAINED. Has it now collapsed and vanished?

At this, he said:

“... The sons of Isaac will come to that city with seventy thousand people; THIS CITY THEY WILL COME TO IS KONSTANTINIYYE. THIS ARMY WILL TAKE THAT CITY.

IN TAKING IT, THEY WILL USE NEITHER SWORDS NOR ARROWS NOR ANY OTHER WEAPON …THEY WILL DEFEAT AND DEVASTATE THOSE WHO OPPOSE THEM BY RECITING THE DECLARATION OF THE ONENESS OF ALLAH. SUCH THAT THEY WILL ALL RECITE THE TAKBIR WITH ONE VOICE. THEY WILL FIRST SAY LA ILAHE ILLALLAH [THERE IS NO DEITY EXCEPT ALLAH], AND THEN ALLAHU AKBAR [ALLAH IS THE GREATEST] … AT THIS, THE SEA PART OF THE CITY WILL BEGIN FALLING DOWN. THEY WILL REPEAT THE SAME DECLARATION OF THE ONENESS OF ALLAH AND ONCE AGAIN RECITE THE TAKBIR; THE REMAINING PART WILL THEN FALL DOWN ENTIRELY. THOSE HEARING THAT SOUND WILL THINK THE DAY OF RECKONING HAS COME. HOW CAN THAT CITY NOT THEN FALL DOWN? HOW CAN IT NOT COLLAPSE?  THEY WILL BEGIN REMEMBERING ALMIGHTY ALLAH FOR A THIRD TIME; AND THE CITY WILL OPEN UP ENTIRELY, AND THEY WILL ENTER IN.

(Muhammediye Kitabı, Yazıcıoğlu Muhammed, p. 442-443)