Fatwa ID: 02379
Answered by: Maulana Maruf Ahmed
Assalamualaykum Sometimes after I have taken a ghusul from ejaculation, I find that mazi comes out late afterwards, sometimes maybe even half an hour to an hour later. I do ghusl and pray my salah and I find out afterwards that mazi came out during the prayer but I did not feel it or realise until a while after, is my prayer valid or should it be repeated?
بسم الله الرحمن الرحيم
Generally, the emitting of madhi invalidates wudu.
If you are unsure or in doubt whether madhi was released during salah or not, then your wudu will remain intact. This is based on the principle;
“Certainty is not annulled by doubt”. 
However, if you have reasonable conviction that madhi was released during salah, then you wudu, and subsequently your salah, will have become invalidated. Hence, salah must be repeated.
Only Allah knows best.
Maulana Maruf Ahmed
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham.
Ibn Abideen: Radd al-Muhtar, vol.1 pg. 280, Dar al-Kutub al-Ilmiyyah, Beirut