Masturbation: With the Hand or With Anything Else

The noble Shaykh, Muhammad bin 'Uthaimin, was asked about this habit, and he answered:

Masturbation, inciting ejaculation with the hand or with any other object, is unlawful as proven by the Qur'an, the Sunnah, and sound reasoning. Allah Almighty says:
"And those who guard their chastity (i.e. private parts, from illegal sexual acts). Except from their wives or (the slaves) that their right hands possess, for then, they are free from blame; But whoever seeks beyond that, then those are the transgressors."

When a person seeks to satisfy his lust with other than his wife or (legal) slave, but with someone else, then he is "seeking beyond" what is allowed, and hence among the "trespassers.

And from the Sunnah, the Messenger of Allah ﷺ said:
"O group of young people, whoever among you is able to marry, then let him marry, for marriage is more modest for the eyes and more safe for the private part. If someone is not able, then let him fast, for it is a shield (against desire)."

Here, the Messenger of Allâh advised that the person who cannot marry to fast; had masturbation been lawful, he would have advised it instead. Since the Messenger of Allah ﷺ did not offer masturbation as a choice when it was a perfect situation for him to do so, we can thus conclude that it is unlawful. Moreover, many doctors have spoken about some of the ill effects of masturbation - ill effects to the body, to one's desire, to one's thinking; it might even prevent someone from marrying, for if someone is satisfying his lusts in this way, he may turn away from marriage.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 8 Pages 297-298