Harry did not cast any spell. His wand worked on its own.
Firstly, the scene:
As the pain from Harry’s scar forced his eyes shut, his wand acted of its own accord. He felt it drag his hand around like some great magnet, saw a spurt of golden fire through his half-closed eyelids, heard a crack and a scream of fury.
Harry Potter and the Deathly Hallows, Chapter 4 - The Seven Potters:
Harry admitting that his wand acted on it's own:
"Wait till it gets out yeh did it again, Harry,” said Hagrid. “Escaped him, fought him off when he was right on top of yeh!”
“It wasn’t me,” said Harry flatly. “It was my wand. My wand acted of its own accord.”
Harry Potter and the Deathly Hallows, Chapter 5 - Fallen Warrior:
“I believe that your wand imbibed some of the power and qualities of Voldemort’s wand that night, which is to say that it contained a little of Voldemort himself. So your wand recognized him when he pursued you, recognized a man who was both kin and mortal enemy, and it regurgitated some of his own magic against him, magic much more powerful than anything Lucius’s wand had ever performed. Your wand now contained the power of your enormous courage and of Voldemort’s own deadly skill: What chance did that poor stick of Lucius Mal- foy’s stand?”
Harry Potter and the Deathly Hallows, Chapter 35 - King’s Cross