Lex Cincia de Muneralis passed by the Plebeian Council. Its intent was the reform of the Roman legal system. The law was introduced by the Tribune Marcus Cincius Alimentus. The provision that most interests us, deals with compensation for lawyers representing their clients. In short, lawyers were forbidden from accepting compensation for legal representation.

1. Titus Livy. History of Rome. Book XXXIV.4 Trans. Rev. Canon Roberts. Editor. Ernest Rhys Titus. Publisher. J.M. Dent & Sons, Ltd. London, 1905.
2. Cicero. de Orat. II.71, ad Att. 1.20

[Look at the context for the law, politics surrounding it, its subsequent evolution all the way through Constantine.] Additional references
Tacitus, Ann. XI.5
Dion Cassius. LIV.18
Tacitus. Ann. XI.7
Suetonius. Nero, 17
Pliny Ep. V.21