The 39-year-old returned an adverse analytical finding of GHRP-2 in an out of competition test on 9 August.
The 2008 Olympic road race champion had been due to ride for BMC Racing at the Vuelta, which starts on Saturday.
He has been suspended by his team with immediate effect and replaced by Loic Vliegen in their Vuelta line-up.
"All riders and staff are held to the highest ethical standard and BMC Racing Team is extremely disappointed to share this news on the eve of the Vuelta," said the team in a statement.
They added "no further action will be taken" until the results of the B sample - which Sanchez has the right to request and attend the analysis of - are provided.
World cycling's governing body, the UCI, said the test was carried out by the Cycling Anti-Doping Foundation, an independent entity in charge of the sport's anti-doping program.
Sanchez has won five stages of the Vuelta and finished runner-up in his home Grand Tour in 2009.
Spain's Samuel Sanchez has been provisionally suspended after testing positive for a growth hormone and is out of this year's Vuelta a Espana.