U.S. Ambassador to the European Union Gordon Sondland will testify to Congress next week that he did not know why United States military assistance to Ukraine was held up nor who ordered it, according to a person with knowledge of Sondland’s testimony before the House next week.

Sondland will say that he "relied on the president’s assurances in good faith and passed these along" when he texted Ambassador to Ukraine Bill Taylor that there was no quid pro quo with Ukraine, the person said. President Donald Trump has urged Ukraine to investigate the son of political rival Joe Biden.

In Sept. 9 text messages given to Congress, Sondland told Taylor that Trump had been "crystal clear" that there was no quid pro quo.

"I think it’s crazy to withhold security assistance for help with a political campaign," Taylor said in a text.

Sondland responded, after speaking with the president, that there were "no quid pro quo’s of any kind."

NBC News has reported that Sondland called Trump before responding to Taylor’s text, expressing concern that there was a quid pro quo to unfreeze military aid for an investigation into Hunter Biden, the son of former vice president Biden, who is a possible challenger to Trump in the 2020 election.

The individual with knowledge says that Sondland "didn’t doubt the president — or he wouldn’t have reassured Taylor."

But the source says Sondland had no independent knowledge about whether or not there really was a quid pro quo and why that military aid, approved by Congress, was being withheld by the administration.

Sondland plans to testify before the House committees conducting the impeachment inquiry Thursday. He initially said he couldn’t testify because the State Department directed him not to. Now he is under subpoena from the House and intends to comply.

The Washington Post Saturday first reported that Sondland plans to testify that his response to Taylor that there was no quid pro quo was directed by Trump after their phone call.

Trump has pressed Ukraine to open an investigation into Biden's son, Hunter, who was a board member of Ukrainian energy firm Burisma from 2014 to 2018. There is no evidence of wrongdoing on the younger Biden's part.

A formal complaint by a whistleblower who's a member of the intelligence community kicked off the formal impeachment inquiry in the house.

The complaint references a July phone call between Trump and Ukrainian president Volodymyr Zelenskiy in which the latter says he's ready to purchase United States military hardware and Trump responds, "I would like you to do us a favor, though" before eventually mentioning Biden's son.