What is the ruling on despairing of Allah’s response to supplication and discouraging people from making du'a?

Question:
May Allah reward you, Honorable Shaykh. This man asks: What is the ruling when a man is told, “Call upon your Lord,” and he says, “Allah does not respond to me”? Or when he sees someone calling upon their Lord, he says, “Do not call upon Him and tire yourself out; Allah will not respond to you.” Is this shirk (associating partners with Allah)? And is it minor or major?

Answer:
I seek refuge in Allah. This is not permissible. This is despair of the mercy of Allah the Exalted and Majestic, and it contradicts His saying, the Exalted: This is the ruling of that He responds, and I seek refuge in Allah.
A man from the Children of Israel said, “By Allah, Allah will not forgive so-and-so.” Allah the Glorious and Exalted said, “Who is this who swears an oath against Me, meaning he swears that I will not forgive so-and-so? I have indeed forgiven him, and I have nullified your deed.” A word he spoke regarding Allah contained a wrong against the right of Allah, so He nullified his deed.
So this speech is malicious speech and false speech. It is obligatory for the one who uttered it to repent to Allah the Exalted and Majestic, and not to despair of the mercy of Allah, nor to make people despair of the mercy of Allah, and to tell them that du'a is of no benefit. This is not permissible. This contradicts the command of Allah, is despair of the mercy of Allah, and is a lack of proper conduct with Allah the Glorious and Exalted. What makes you think that Allah does not respond? Are you judging Allah the Exalted and Majestic?
So this is absolutely not permissible. This is very serious. Perhaps this speech will nullify his deed, just as the deed of that man was nullified, about whom the Messenger (peace and blessings be upon him) informed.