Nahj al-Balāgha
Hadith 1889٣٢٤. وَقَالَ (عليه السلام): اتَّقُوا مَعَاصِيَ اللهِ فِي الْخَلَوَاتِ، فَإِنَّ الشَّاهِدَ هُوَ الْحَاكِمُ.
324. Amīr al-mu’minīn, peace be upon him, said: Beware of disobeying Allāh in solitude, because the witness (of that situation) is also the judge.