After finishing my Maghrib sunnahs, I noticed blood which had flowed from a cut on my hands. The last time I had made wudhu was before Asr. I knew the blood couldn't have flowed before then as I had washed my hands immediately before maghrib. Should I repeat Maghrib salah?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
If one is sure the blood was not present while performing ablution for Asr and noticed after Magrib that bleeding had occurred, both prayers would be valid.
The ruling is as follows: doubt will not nullify surety. Based upon this rule, both prayers would be valid and no repetition is required.
Only Allah knows best
Written by Maulana Irfan Ali
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Qawaidul Fiqhiyyah – Third rule page 15