Ma'arif al-Qur'an
Mufti Muhammad Shafi2. With reference to verse 41, Sayyidna Zakariyya's purpose behind requesting a sign (of pregnancy) was to be happy at the prospect and thus be enabled to show their gratitude all along even prior to the actual birth of the child. So, Allah gave him the sign stated above.
The sign given was remarkably suitable to its purpose. His request for a sign was prompted by his wish to show his gratefulness. Now, the sign set for him leaves him incapable of doing anything but this. Even a hundred signs would have not done what this one sign did and, of course, the noble purpose he had on his mind was all too well-accomplished. (Bayn al-Qur'an)The last phrase: (except through gestures) in this verse tells us that in a situation where speech is not possible because of a valid reason, gesture would be deemed as its substitute. Consequently, it appears in a hadith that the Holy Prophet ﷺ asked a maid-servant: این اللہ ؟ I ('Where is Allah?' ). She pointed out towards the sky. He said: 'This maid-servant is a Muslim.' (al-Qurtubi)