Marx never believed that “history repeats itself,” but in a famous quote he said:

“Hegel remarks somewhere that all great world-historic facts and personages appear, so to speak, twice. He forgot to add: the first time as tragedy, the second time as farce.” [Marx, 18th Brumaire of Louis Bonapatre, Chapter 1.]

This seems to come from Engels’ letter to Marx of 3 December 1851:

“it really seems as though old Hegel, in the guise of the World Spirit, were directing history from the grave and, with the greatest conscientiousness, causing everything to be re-enacted twice over, once as grand tragedy and the second time as rotten farce, Caussidière for Danton, L. Blanc for Robespierre, Barthélemy for Saint-Just, Flocon for Carnot, and the moon-calf together with the first available dozen debt-encumbered lieutenants for the little corporal and his band of marshals. Thus the 18th Brumaire would already be upon us.”

 – words quoted almost verbatim by Marx in Eighteenth of Louis Bonapartre.

Marx makes similar points in Critique of Hegel’s Philosophy of Right, Introduction.

Possible sources in Hegel are The Philosophy of Right, §347 and The Philosophy of History, §32-33 though another version of this work published as Introduction to The Philosophy of History, published in 1837, said:

“A coup d’état is sanctioned as it were in the opinion of the people if it is repeated. Thus, Napoleon was defeated twice and twice the Bourbons were driven out. Through repetition, what at the beginning seemed to be merely accidental and possible, becomes real and established.”

but this is hardly the point being made by Marx. See The Philosophy of History, where Hegel contrasts Nature, where “there is nothing new under the Sun,” with History where there is always Development.