It was found that the Haas driver was not holding his maximum lap time in Q2, which was eliminated at the end of a late tour from Charles Leclerc.
But, although he explained to the administrators that he had driven so slowly, because he let go of other cars, in addition to the difficult conditions on the runway, he was still sanctioned.
A statement from the administrators said: "The administrators looked closely at the driver and found that the 20 car was particularly careful not to hinder the other drivers. The driver also noted that he was worried about the rain in these circumstances.
"However, managers have noticed that other drivers have set representative moments at this time and that the delta time is based on 145% of a representative tour.
"That's why the administrators discovered that a violation had been committed, but he considered the driver's concern about the rain and imposed a reprimand."
Brazil's assassination is Magnussen's second season after being awarded one at the Spanish Grand Prix for his role with Leclerc.
If a driver wins three suppressions in a year, he will get a penalty of 10 points, which means that Magnussen will have to avoid any trouble before the end of the final season in Abu Dhabi.
Williams pilot, Sergei Sirotkin, was also rebuked for committing the same offense, although he explained that there was an error during the Delta when he was shown on board.