give p minecraftpotion 1 0 CustomPotionEffectsId1Amplifier1Duration9600

This command will give the nearest player a Potion of Swiftness (Speed) with an amplifier of 1 and a duration of 9600 ticks.

Note: In newer versions of Minecraft, the command syntax has changed. The correct command syntax for the latest version would be: /give @p minecraft:potion{Potion:"minecraft:swiftness",CustomPotionEffects:[{Id:1,Amplifier:1,Duration:9600}]} 1