What is meant by the drinker of Zamzam water attaining their request according to the strength of their belief?

It is not clear whether the aforementioned statement belongs to a scholar or someone else. However, the virtue of Zamzam water is established by the hadith: "Zamzam water is for what it is drunk for." So, whoever drinks Zamzam and supplicates to Allah for a need to be fulfilled, Allah will answer him, with the importance of the servant's supplication being from a present heart and with certainty of a response, as the Prophet (peace be upon him) said: "Supplicate to Allah while you are certain of being answered, for Allah does not answer a supplication from a heedless, inattentive heart." The power of supplication is commensurate with the strength of the servant's certainty. Ibn al-Qayyim likened supplication to a sword, its effect being determined by the strength of the supplicant's faith.