Answered by Ustadh Salman Younas
Question: Is it permissible to personally masturbate prior to intercourse for the purpose of lasting longer during intercourse so that I can fulfill/satisfy my wife?
Answer: assalamu `alaykum
Self-masturbation is not permissible. Thus, one cannot self-masturbate for the reason described in the question.
As for when this act is performed by one’s spouse, namely mutual masturbation, it would be permissible.
[Ibn `Abidin, Radd al-Muhtar]
Checked & Approved by Faraz Rabbani