﴿ قَالَ رَبِّ ٱجْعَل لِّىٓ ءَايَةًۭ ۚ قَالَ ءَايَتُكَ أَلَّا تُكَلِّمَ ٱلنَّاسَ ثَلَٰثَ لَيَالٍۢ سَوِيًّۭا ﴾

“[Zachariah] prayed: "O my Sustainer! Appoint a sign for me!" Said [the angel]: "Thy sign shall be that for full three nights [and days] thou wilt not speak unto men.”

سَوِيًّا means healthy. This word has been added here to denote that Sayyidna Zakariyya's (علیہ السلام) loss of speech for three days was not due to any illness, for during this entire period of three days he could utter words of prayer and supplication. This special condition was a miracle and a sign from Allah Ta’ ala about his wife's pregnancy.