It is narrated from Sayyiduna Abbas bin Abd al-Muttalib (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There is no retaliation for an ordinary head wound (ma'mumah), a belly wound (ja'ifah), or a head wound from which bone fragments come out (munaqqilah)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16101