كتاب المكاتب
The Mukatab

Malik said, "If people are together in one kitaba, their master cannot free one of them without consulting his companions who are with him in the kitaba and obtaining their consent. If they are young, however, their consultation means nothing and it is not permitted to them. That is because a man might work for all the people and he might pay their kitaba for them to complete their freedom. Their master approaches the one who will pay for them and their rescue from slavery is through him. He frees him and so makes those who remain unable to pay. He does it intending benefit and increase for himself. It is not permitted for him to do that to those of them who remain. The Messenger of Allah, may Allah bless him and grant him peace, said, 'There must be no harm nor return of harm.' This is the most severe harm."

Malik said about slaves who wrote a kitaba together that it was permitted for their master to free the old and exhausted of them and the young when neither of them could pay anything, and there was no help nor strength to be had from any of them in their kitaba.

USC-MSA web (English) reference : Book 39, Hadith 13