Does a small trace of feces (the size of a fingernail or less) at the anus after repeated istinja' nullify wudu'?

One must ensure purity from urine and feces. If the trace of feces is continuous, the ruling is incontinence ( سلس ). You should wash the affected area thoroughly, tie a cloth tightly over it, and perform ablution after the entry of the time for each obligatory prayer and as many supererogatory prayers as you wish. The exit of something, even during prayer, does not matter. If the discharge stops before the time of the obligatory prayer expires, you should delay the prayer until it ceases, then cleanse yourself (istinja') and pray, even if you miss the congregational prayer. This is unless you fear the time will expire, in which case you pray in the first manner.