Tafsir of Al-Anbiya' 21:89

"And [mention] Zechariah, when he called out to his Lord..."

Meaning: And mention his account, peace be upon him, "My Lord, do not leave me alone [childless]," meaning: solitary, without a child to inherit from me, as is indicated by the appending statement of the Exalted, "and You are the best of the inheritors."

Had the intended meaning been merely [a lack of] a child to accompany and assist him, it would have been said, "And You are the best of the helpers." The intended meaning of His saying, "And You are the best of the inheritors," is: You are the best of those who remain alive after the dead have passed. In this is a praise of Him, the Exalted, for [His] eternal existence, and an allusion to the perishability of all other living beings. In that, there is also a seeking of the downpour of the clouds of His grace, the Almighty and Majestic.

It has been said that by this he intended to refer the matter back to Him, the Glorified, as if he had said: If You do not grant me a child to inherit from me, then You are the best Inheritor, and You are sufficient for me. This has been objected to on the grounds that it does not befit the station of supplication, for among the etiquettes of the supplicant is that he should pray with earnestness, diligence, and firm resolve. It is recorded in the Sahihayn from the Messenger of Allah (may Allah bless him and grant him peace) that he said: "When one of you supplicates, he should not say: 'O Allah, forgive me if You will, have mercy on me if You will, provide for me if You will.' Rather, he should be resolute in his request, for Allah, the Exalted, does whatever He wills, and there is no one who can compel Him." In a narration in Sahih Muslim: "But let him be firm in the request and firm in the desire, for there is nothing that Allah, the Exalted, gives that is too great for Him."

It is possible to respond by saying: This is not of the same category as "provide for me if You will," because the intent here is nothing more than manifesting contentment and reliance upon Allah, the Almighty and Majestic, in the event that his supplication is not answered—whereas that [prohibited phrasing] is not the intended meaning of "provide for me if You will." So contemplate this.