Musannaf Ibn Abi Shayba 13:882

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Ayyub ibn ‘A’idh al-Ta’i, from ‘Amir, he said: The teacher shall not stipulate a condition [for payment]. If something is gifted to him, he should accept it.