Quran 15:99 from Surah Al-Hijr on Divine Marvels.
وَٱعۡبُدۡ رَبَّكَ حَتَّىٰ يَأۡتِيَكَ ٱلۡيَقِينُ
English translation: And worship your Lord until comes to you the certainty
Transliteration: wa-u'bud rabbaka hatta yatiyaka al-yaqinu