The article of impeachment introduced by Democrats in the House on January 11th specifically mentions parts of President Trump's January 6th speech (emphasis mine):

Shortly before the Joint Session commenced, President Trump, addressed a crowd at the Ellipse in Washington, DC. There, he reiterated false claims that "we won this election, and we won it by a landslide". He also willfully made statements that, in context, encouraged - and foreseeably resulted in - lawless action at the Capitol, such as: "if you don’t fight like hell you’re not going to have a country anymore". Thus incited by President Trump, members of the crowd he had addressed, in an attempt to, among other objectives, interfere with the Joint Session’s solemn constitutional duty to certify the results of the 2020 Presidential election, unlawfully breached and vandalized the Capitol, injured and killed law enforcement personnel, menaced Members of Congress, the Vice President, and Congressional personnel, and engaged in other violent, deadly, destructive, and seditious acts.

The full context of this quote, and the transcript of Trump's speech in full, can be found here:

Our brightest days are before us, our greatest achievements still wait. I think one of our great achievements will be election security because nobody until I came along, had any idea how corrupt our elections were. And again, most people would stand there at 9:00 in the evening and say, “I want to thank you very much,” and they go off to some other life, but I said, “Something’s wrong here. Something’s really wrong. Can’t have happened.” And we fight. We fight like Hell and if you don’t fight like Hell, you’re not going to have a country anymore.

As far as the allegations by House Democrats of incitement of insurrection go, this extract from his speech seems to be the main focus, although the article of impeachment is not solely in response to his speech on Jan 6th, and also mentions his prior conduct such as his phone call with the Georgia Secretary of State on January 2nd.

The article of impeachment introduced by Democrats in the House on January 11th specifically mentions parts of President Trump's January 6th speech (emphasis mine):

Shortly before the Joint Session commenced, President Trump, addressed a crowd at the Ellipse in Washington, DC. There, he reiterated false claims that "we won this election, and we won it by a landslide". He also willfully made statements that, in context, encouraged - and foreseeably resulted in - lawless action at the Capitol, such as: "if you don’t fight like hell you’re not going to have a country anymore". Thus incited by President Trump, members of the crowd he had addressed, in an attempt to, among other objectives, interfere with the Joint Session’s solemn constitutional duty to certify the results of the 2020 Presidential election, unlawfully breached and vandalized the Capitol, injured and killed law enforcement personnel, menaced Members of Congress, the Vice President, and Congressional personnel, and engaged in other violent, deadly, destructive, and seditious acts.

The wider context of this quote, and the transcript of Trump's speech in full, can be found here:

Our brightest days are before us, our greatest achievements still wait. I think one of our great achievements will be election security because nobody until I came along, had any idea how corrupt our elections were. And again, most people would stand there at 9:00 in the evening and say, “I want to thank you very much,” and they go off to some other life, but I said, “Something’s wrong here. Something’s really wrong. Can’t have happened.” And we fight. We fight like Hell and if you don’t fight like Hell, you’re not going to have a country anymore.

As far as the allegations by House Democrats of incitement of insurrection go, this extract from his speech seems to be the main focus, although the article of impeachment is not solely in response to his speech on Jan 6th, and also mentions his prior conduct such as his phone call with the Georgia Secretary of State on January 2nd.