The word(s) "slacken" appears 2 time(s) in 2 verse(s) in Quran in Mohsin Khan translation.

(1) "Go you and your brother with My Ayat (proofs, evidence, verses, lessons, signs, revelations, etc.), and do not, you both, slacken and become weak in My Remembrance.

(سورة طه, Taa-Haa, Chapter #20, Verse #42)
(Arabic, Transliteration, Urdu (Ahmed Ali), Urdu (Jalandhry), Farsi, Yusuf Ali, Shakir, Picthal, Mohsin Khan, French, Spanish, Indonesian, Malay, German, Bosnian, Russian, Dutch, Italian, Portuguese, Tafsir - Jalal ad-Din, Tafsir - King Fahad Quran Complex)

(2) They (i.e. the angels) glorify His Praises night and day, (and) they never slacken (to do so).

(سورة الأنبياء, Al-Anbiyaa, Chapter #21, Verse #20)
(Arabic, Transliteration, Urdu (Ahmed Ali), Urdu (Jalandhry), Farsi, Yusuf Ali, Shakir, Picthal, Mohsin Khan, French, Spanish, Indonesian, Malay, German, Bosnian, Russian, Dutch, Italian, Portuguese, Tafsir - Jalal ad-Din, Tafsir - King Fahad Quran Complex)