It is mentioned in hadith:
ناكح اليد معلون.
One who marries with hands (masturbator) is accursed.
It is also mentioned in another hadith that the finger of a masturbator shall be pregnant on the day of judgment i.e. he will be humiliated in front of all the people. These hadith speak openly that it is haram.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband