What is the ruling on expelling residual seminal fluid from a wet dream by hand?

Expelling semen by hand (masturbation) is forbidden by Islamic law. Malik and Al-Shafi'i cited as evidence the Almighty's saying: "And those who guard their private parts Except from their wives or those their right hands possess, for indeed, they will not be blamed But whoever seeks beyond that, then those are the transgressors." As for milking the penis or shaking it to expel drops of semen retained after a wet dream, this is not considered forbidden masturbation. Rather, it falls under istibra' (cleansing after urination/ejaculation), which varies according to people's needs.