Musannaf Ibn Abi Shayba 18:883

Abu Bakr narrated to us, he said: Isma'il ibn Ibrahim narrated to us, from Muhammad ibn Ishaq, he said: 'Abd al-Rahman ibn Abi Za'idah narrated to me, from Nafi' ibn Jubayr, from Ibn 'Abbas, he said: Four thousand is added to the blood money of the one killed during the sacred months. And for the one killed within the Sacred Precinct (Haram), four thousand is added to his blood money. The value of the blood money of the one in the Sacred Precinct is twenty thousand.