The Quran

قالَ رَبِّ اجعَل لي آيَةً ۖ قالَ آيَتُكَ أَلّا تُكَلِّمَ النّاسَ ثَلاثَةَ أَيّامٍ إِلّا رَمزًا ۗ وَاذكُر رَبَّكَ كَثيرًا وَسَبِّح بِالعَشِيِّ وَالإِبكارِ

Muhammad Asad

[Zachariah] prayed: "O my Sustainer! Appoint a sign for me!" Said [the angel]: "Thy sign shall be that for three days thou wilt not speak unto men other than by gestures.¹ And remember thy Sustainer unceasingly, and extol His limitless glory by night and by day."

According to Abu Muslim (quoted with approval by Razi), Zachariah was merely enjoined not to speak to anyone during the period of three days, and not struck dumb as in the New Testament narrative (Luke i, 20-22): thus the "sign" was purely spiritual, and was to consist in Zachariah's utter self-abandonment to prayer and contemplation.