Quran 44:59 from Surah Ad-Dukhaan on Divine Marvels.
فَٱرۡتَقِبۡ إِنَّهُم مُّرۡتَقِبُونَ
English translation: So watch indeed they (too are) watching
Transliteration: fa-ir'taqib innahum mur'taqibuna