It's from Season 26, ep 8, "The Curse of Fenric", specifically part 3.

The Doctor reveals that faith or strong belief repels "hemavores" or vampires and repels a bunch by closing his eyes and believing in something. A higher ranking Soviet soldier that landed with a group of such soldiers in England during the second world war (reasons I forget) ends up separated from most of his troops who are still on the beach and instead with the Doctor, his companion Ace, and a priest. He insists he must go back for his men. Ace (the Doctor's present companion) asks him to teach the soldier "the singing" to scare them off. The Doctor states he either really believes in something or he doesn't to which the Soviet soldier replies he believes in the revolution. Some scene-cuts later the soldier, leaving the church is confronted by a group of these creatures and pulls out the pin, focusing on his belief in the revolution and they clutch themselves and start screaming as he walks slowly through them.

Someone made an edit with the Soviet anthem playing layered over it but in the original show it's just dramatic music stings with some ringing noises like those that accompanied the Doctor's actions earlier, no anthem plays.

And as mentioned elsewhere a priest in the episode can't repel them because he lost his faith due to the war.