﴿ يَٰيَحْيَىٰ خُذِ ٱلْكِتَٰبَ بِقُوَّةٍۢ ۖ وَءَاتَيْنَٰهُ ٱلْحُكْمَ صَبِيًّۭا ﴾

“[And when the son was born and grew up, he was told,] "O John! Hold fast unto the divine writ with [all thy] strength!" - for We granted him wisdom "while he was yet a little boy,”

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.