Traces of the nerve agent allegedly used to poison Russian politician Alexei Navalny were found on a bottle in the hotel room where he stayed before falling ill, his team has said.
Mr Navalny collapsed on a flight in Siberia in August. Germany says he was poisoned with a Novichok nerve agent.
It was previously thought that he might have been targeted at the airport.
"Now we understand: It was done before he left his room to reach the airport," a post on his Instagram account says.
Mr Navalny, Russia's most high profile opposition figure and a leading critic of Russian President Vladimir Putin, is currently receiving treatment in the German capital Berlin.
His team alleges he was poisoned on the orders of Mr Putin.
The Kremlin denies any involvement in the case and says its doctors found no evidence that a nerve agent was used.
No official investigation has been launched in Russia, although the head of Mr Navalny's anti-corruption foundation Ivan Zhdanov tweeted that, as part of preliminary enquiries, an investigator from Tomsk - where the attack happened - wanted to question two of the foundation's employees who were with Mr Navalny when he fell ill.
The case has caused a diplomatic rift between Berlin and Moscow. The European Parliament on Thursday approved a non-binding resolution urging the EU to call for an international investigation into the poisoning.
What does the video show?
The video posted on Mr Navalny's Instagram account shows members of his team in a hotel room in the Siberian city of Tomsk after news of his poisoning emerged.
The post says they were there to gather potential evidence from the hotel to send to Mr Navalny's medical team in Germany because they did not trust the Russian authorities.
Several empty water bottles can be seen in the clip. They are among a number of items bagged by individuals wearing gloves.
"It is precisely on the bottle from the Tomsk hotel room that a German laboratory found traces of Novichok," the post says.
Russia's Proyekt news site quoted one of Novichok's creators, Vladimir Uglev, as saying that the poison was unlikely to have been in the water as the fact Mr Navalny survived suggests he had only skin contact with it.
The German authorities have not commented on the alleged finding.
Earlier this week, Mr Navalny posted once for the first time since he fell ill. He said he was now able to breathe unaided.
His spokeswoman said he intended to return to Russia.
What happened to Navalny?
The Russian anti-corruption campaigner became ill during the flight from Tomsk to Moscow on 20 August, and the plane made an emergency landing in the city Omsk. Russian officials were persuaded to allow him to be airlifted to Germany two days later.
After carrying out tests, Germany said there was "unequivocal proof" that Mr Navalny had been poisoned with a Novichok nerve agent.
Labs in Sweden and France later confirmed the findings, according to the German government.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said on Thursday it would also carry out tests at the request of Germany.
A nerve agent from the Novichok group was also used to poison Russian ex-spy Sergei Skripal and his daughter in Salisbury, in England, in 2018. They both survived, but a local woman, Dawn Sturgess, died after coming into contact with the poison.
Britain accused Russia's military intelligence of carrying out that attack. Twenty countries expelled more than 100 Russian diplomats and spies in response. Moscow denied any involvement.