Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.050B.

Section : If a man squeezes his penis to prevent ejaculation.

This also makes ghusl unnecessary. This is based on the hadith which states that ghusl is required if the sperm can be seen. But, if the person walks and cannot control his ejaculation, he must perform ghusl.