How to attain full purity? Please see details

Assalamoalaikum. I have a question in my mind from past few days. The question is that after having sexual intercourse or, say, after masturbating one does ghusl to attain purity. But what if some semen remained in the urinary tract of penis and after a small time of doing ghusl he do pee and that semen came out that time. What would be done in this situation? Do he needs to do ghusl again? Please give a nice answer with some relevant reference, if any. Jazakallah hu khair.