Sheikh `Abd al-Rahmân al-`Ajlân, lecturer at the Grand Mosque in Mecca
If someone masturbates during the day in Ramadan and achieves orgasm or ejaculates semen, he has to sincerely repent to Allah for his sin and make up the broken fast. Further expiation will not be necessary in this case, because that is only in the case of actual sexual intercourse.
If a person begins to masturbate and stops before achieving orgasm or ejaculation, then his fast is intact. The appearance of pre-semenal fluid does not break the fast.
And Allah knows best.
Source: Islam Today