Answered by Ustadh Tabraze Azam
Question: As-Salamu Alaykum, would it be permissible to perform wudu (in the sink) then perform istinja and the rest of the ghusl in the shower – if impurities (minor) exit after performing or during wudu and the person is going to prayer later, can they continue and perform wudu before they begin the prayer?Answer: Wa alaikum assalam wa rahmatullah,
I pray that you are in the best of health and faith, insha’Allah.
 Performing the ablution (wudu) first would be contrary to the sunna. As the sunna is to first clean your private parts, then to perform a complete wudu, and then to take the bath (ghusl).
 If you perform a ghusl after the wudu, then you are in a state of purification. You do not need to repeat anything.
[Shurunbulali, Maraqi al-Falah]
And Allah alone gives success.