According to the marvel wiki, Mystique is Nightcrawler's mother.
When was this declared in the comic book? I do know that they mentioned it in both the 1990 X-Men cartoon and X-Men:Evolution series.
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This was confirmed in X-Men Unlimited #4, published in 1994.
As Marvel's own site explains:
At the time disguised as the Baroness Wagner, Mystique's true identity was revealed after she gave birth to Kurt, and the pair were chased by an angry mob. Fleeing to the nearby falls, Mystique threw the infant Kurt over the edge and fled. Kurt was saved by his natural father, the enigmatic being known as Azazel, and given into the care of Margali Szardos, a sorceress and gypsy queen.
Specifically, Graydon Creed hires Gunther Reinhold to find out more about Creed's past/origin and learns that he and Kurt are both sons of Mystique. Creed later reveals this to Kurt.