This involves bleeding due to a wound, cupping or a nosebleed, and regardless of whether the amount of blood is small or large.
Said al-Hassan, “The Muslims still prayed even while wounded.” (Related by al-Bukhari.) He also reported, “Ibn ‘Umar squeezed a pimple until it bled, but he did not renew his ablution. Ibn Abi ‘Uqiyy spat blood and continued his prayer. ‘Umar ibn al-Khattab prayed while blood was flowing from him. ‘Ibbad ibn Bishr was hit with an arrow while praying, but continued his prayers.” (Related by Abu Dawud, Ibn Khuzaimah, and al-Bukhari in mu ‘allaq form.)