What is the ruling on praying in a garment that has been stained by madhi (pre-seminal fluid) and has dried?

Madhi is a thin, sticky, white fluid that is discharged when desire is aroused. It is impure (najis). What contaminates the body must be washed, and it is sufficient to sprinkle water on what contaminates clothing. If one prays knowing that madhi and its impurity are on his clothes, his prayer is invalid due to the absence of the condition of clothing purity. However, if one was ignorant of its impurity, or forgot to purify it, or did not know of its presence until after the prayer, then his prayer is valid.