The source did not give any further details.
Santander declined to comment while Goldman Sachs and Credit Suisse were not immediately available for comment.
Earlier this year, Santander had submitted a non-binding bid for Citibanamex, another source with direct knowledge of the process said, who added that binding bids were expected later this year.
Spanish newspaper Expansion, which first reported the news on Tuesday, said that the sale process could be outlined before the end of the summer.
Last Friday, Citigroup Latin America Chief Executive Ernesto Torres Cantu said that the bank did not expect to reach a deal to sell its Mexican consumer banking arm Citibanamex until January 2023.
Cantu said the bank was engaged with potential buyers, but declined to provide more details about the stage of the sale process.
Citi aims to sell Citibanamex's entire consumer and middle market businesses to a sole buyer, Cantu told a media briefing.
(Reporting by Jesús Aguado; editing by Inti Landauro and Louise Heavens)