Musannaf Ibn Abi Shayba 13:2199

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Khalid ibn ‘Adi, that he wrote to ‘Umar ibn ‘Abd al-‘Aziz regarding a man who made a *kitabah* contract with a slave of his, and stipulated upon him a share of his inheritance. He wrote back that no one has a condition that diminishes or divides any of the obligatory shares [ordained] by Allah.