Selections from The Sayings and Preaching of Amir al-Muminin Ali ibn Abi Talib (Peace Be Upon Him) Including His Replies to Questions and Maxims Expressed for Various Purposes.Translated by Sayyid Ali Reza
وَقَالَ (عليه السلام):
Amir al-mu'minin, peace be upon him, said:
اتَّقُوا مَعَاصِيَ اللهِ فِي الْخَلَوَاتِ،
Beware of disobeying Allah in solitude,
فَإِنَّ الشَّاهِدَ هُوَ الْحَاكِمُ.
because the witness (of that situation) is also the judge.