The Letter Qaf 50:18. Juz 26, page 519.
مَّا يَلْفِظُ مِن قَوْلٍ إِلَّا لَدَيْهِ رَقِيبٌ عَتِيدٌ١٨
He uttereth no word but there is with him an observer ready.
Qaf 50:18, translated by Marmaduke Pickthall