Is it necessary to change the dressing over an external wound on the lower abdomen before every prayer? And does the quantity of blood, whether much or little, matter if the blood is on the dressing and not on the body or clothing?

It is obligatory for someone with a wound that does not stop bleeding to bandage it, and he is not required to re-tie the bandage for every prayer. A person suffering from incontinence, a woman with abnormal uterine bleeding (istihada), someone with a wound that does not stop bleeding, or one with chronic nosebleeds, must wash the area contaminated with impurity and bandage it with a clean cloth to prevent the discharge as much as possible. It is stated in Matalib Uli al-Nuha: "If it is not possible to bandage it... he prays according to his condition. If blood then overwhelms [the bandage] and drips, his purification is not invalidated, due to the impossibility of preventing it. He is not required to repeat ablution, nor to re-bandage for every prayer, as long as there is no negligence." The blood on the bandage is excused due to hardship. If the wound is not actively bleeding, the blood on the dressing is excused if it is minor; however, if it is significant, it must be removed, unless its removal would cause harm, in which case it is excused.