Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
It has been authentically established in the hadiths that the prayers and greetings of the Muslims upon the Prophet ﷺ reach him, and that Allah تعالى returns his soul to him so that he may respond to whoever sends greetings of peace upon him.
The Prophet ﷺ said:
“Allah has angels traveling throughout the earth who convey to me the greetings of peace from my Ummah.” Narrated by An-Nasa’i.
And he ﷺ said:
“There is no one who sends greetings of peace upon me except that Allah returns my soul to me so that I may respond to his greeting.” Narrated by Abu Dawud; Al-Albani graded it as good.
Whatever has been authentically established from the Prophet ﷺ must be believed and affirmed, whether we know how it occurs or not, because the matters of the Barzakh are among the unseen matters whose true nature cannot be comprehended by human reason.
Ibn Hajar رحمه الله mentioned that the difficulty concerning how the Prophet ﷺ responds to the large number of people who send greetings of peace upon him cannot be resolved by comparing the conditions of the Barzakh with the conditions of this worldly life. This is because the conditions of the Hereafter and the Barzakh cannot be comprehended by human intellect, nor can they be measured according to what is customary in this worldly life.
Allah تعالى is also capable of conveying the greetings of the Prophet’s ﷺ Ummah to him and enabling him to respond to them, regardless of how numerous they may be. Such a matter is not beyond the power of Allah تعالى.
As for the Prophet ﷺ giving his Ummah to drink at the Hawd, it is not correct to measure the conditions of the Hereafter according to human abilities and circumstances in this worldly life. Rather, we believe in what has been authentically reported from the Prophet ﷺ concerning the Hawd, intercession, and other matters, and we entrust the manner in which these things occur to Allah تعالى.
Therefore, what is required is to believe in what has been authentically reported from the Prophet ﷺ and not to question its authenticity merely because we find its manner of occurrence or the large number of people difficult to comprehend. The power of Allah تعالى is not limited by the limitations that restrict human abilities.
And Allah تعالى knows best.