Son and party vice president Rahul Gandhi said she is "much better" and there is "nothing to worry".
D S Rana, Chairman (Board of Management) of the Ganga Ram Hospital said that Gandhi, 70, is under observation.
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"Sonia Gandhi was brought to the hospital at 5 PM today. She has been found to be suffering from stomach upset and has been admitted for observation," said Rana in a statement.
Rahul Gandhi later tweeted, "Ma was in Shimla and caught a stomach bug so we got her back. Nothing to worry, she's much better. Thanks for the tremendous love and concern."
Congress spokesperson Randeep Singh Surjewala said on Twitter: "Congress President Sonia Gandhi was indisposed with a minor issue and is absolutely fine and healthy. We thank all well wishers for the concern."
A team of doctors from the hospital was deputed to bring her back from Shimla.