But when ye shall see the abomination of desolation, spoken of by Daniel the prophet, standing where it ought not, (let him that readeth understand,) then let them that be in Judaea flee to the mountains

Mark 13:14

The referenced passage from Daniel 11:21-31 --

And in his estate shall stand up a vile person, to whom they shall not give the honour of the kingdom: but he shall come in peaceably, and obtain the kingdom by flatteries. And with the arms of a flood shall they be overflown from before him, and shall be broken; yea, also the prince of the covenant. And after the league made with him he shall work deceitfully: for he shall come up, and shall become strong with a small people. He shall enter peaceably even upon the fattest places of the province; and...he shall scatter among them the prey, and spoil, and riches: yea, and he shall forecast his devices against the strong holds, even for a time...

Then shall he return into his land with great riches; and his heart shall be against the holy covenant; and he shall do exploits, and return to his own land....therefore he shall be grieved, and return, and have indignation against the holy covenant: so shall he do; he shall even return, and have intelligence with them that forsake the holy covenant. And arms shall stand on his part, and they shall pollute the sanctuary of strength, and shall take away the daily sacrifice, and they shall place the abomination that maketh desolate.