Please consider all the possibilities when you answer this answer. For a while now, the devil has infected me with this question, and it is making me uneasy. My question is: What is the proof that Allaah does not lie at all? In other words, what is the proof that Allaah is Honest and Truthful? What is the proof that He does not deceive us at all in the Quran?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

Firstly, you should know that experiencing such evil thoughts is the result of satanic incitements employed in the devil's evil attempts to control the believer in order to undermine his religiosity. In fact, it is a sign of despair on part of the devil. The following has been reported on the authority of Ibn ʻAbbaas : “A man came to the Prophet, sallallaahu ʻalayhi wa sallam, and said: ‘O Messenger of Allaah! One of us has thoughts of such nature that he would rather be reduced to charcoal than speak about them.’ He, sallallaahu ʻalayhi wa sallam, said, ‘Allaah is The Most Great, Allaah is The Most Great; Allaah is The Most Great. Praise be to Allaah, Who has reduced the guile of the devil to evil prompting.’” [Ahmad and Abu Daawood]

Secondly, the fact that you are worried and concerned because of experiencing such evil thoughts is, actually, a good sign. It indicates your goodness. You are advised to keenly ward off such evil thoughts and satanic whisperings and seek refuge with Allaah from them. Abu Hurayrah narrated that a group of people went to the Prophet, sallallaahu ʻalayhi wa sallam, and said, “Verily we perceive in our minds that which every one of us considers too grave to express.” He, sallallaahu ʻalayhi wa sallam, said, “Do you really perceive it?” They said, “Yes.” Upon this he, sallallaahu ʻalayhi wa sallam, remarked, “That is pure faith.” [Muslim]

Moreover, Abu Hurayrah reported that the Prophet, sallallaahu ʻalayhi wa sallam, said, “Satan comes to one of you and says, ‘Who created such-and-such?’ till he says, ‘Who has created your Lord?’ So, when he inspires such a question, one should seek refuge with Allaah and give up such thoughts.” [Al-Bukhari and Muslim]

Thirdly, the belief in Allaah as the Lord entails believing that He is far Exalted above all deficiencies and imperfection. Allaah, The Exalted, says (what means): {Exalted is your Lord, the Lord of might, above what they describe. And peace upon the messengers. And praise to Allaah, the Lord of the worlds.} [Quran 37:180-182]

Ibn Al-Qayyim commented on these verses: “Allaah deemed Himself Exalted above any attribute of imperfection ascribed to Him and conferred His peace on His Messengers, who ascribe the attributes of perfection befitting Him to Him, far above any imperfection and defects. Then, He praised Himself; He is the One to whom the attributes of perfection are ascribed for which He is worthy of praise; He is far exalted above any aspect of imperfection contradicting His perfect praise...”  [Jalaa’ Al-Afhaam]

So, how can He be Lord if He lies and deceives His slaves? Allaah, The Exalted, says (what means): {Allaah - there is no deity except Him. He will surely assemble you for [account on] the Day of Resurrection, about which there is no doubt. And who is more truthful than Allaah in statement.} [Quran 4:87]

Fourthly, honesty and truthfulness are among the attributes of human perfection; Allaah created human beings and inculcated these good attributes in them. He Who grants perfection is worthier of manifesting perfection, as befitting Him, for a greater reason.

Ibn Taymiyyah wrote: “The speech of Allaah should be deemed exalted above any aspect of imperfection or shortcomings; He is worthy of ultimate perfection in His essence, attributes, and actions (as befitting His Majesty). It cannot be perceived that He is devoid of any attribute of perfection pertaining to life, knowledge, power, and speech. Allaah, The Necessarily Existent (Waajib al-Wujood), The Timelessly Eternal (al-Qadeem) and The Pre-eternal (al-Azali) – existing without any prior nonexistence and without any beginning – is worthier of the attributes of perfection than the created. All aspects of perfection attributed to the created, The Creator is The One Who bestows them upon them. He Who imparts perfection is worthier of manifesting it for a greater reason.” [Majmoo’ Al-Fataawa]

Fifthly, Allaah, The Exalted, highlighted in the Quran that if the Prophet, sallallaahu ʻalayhi wa sallam, hypothetically, had attributed a lie to Him, He would have severely punished him for that. However, this never happened. This proves that the Quran was revealed by Allaah to His Messenger; it testifies to its authenticity and the truthfulness of the Prophet, sallallaahu ʻalayhi wa sallam. Allaah, The Exalted, says (what means): {And if Muhammad had made up about Us some [false] sayings, We would have seized him by the right hand, then We would have cut from him the aorta.} [Quran 96:44-46]

Ibn Katheer wrote, “This means that if Muhammad forged something against Us, as they claim, and added or removed anything from the Message or said anything from himself while attributing it to Us, then We would surely be swift in punishing him. And of course, Muhammad did not do any of this (as the disbelievers claimed).”

As-Saʻdi said:

“Had the Prophet, sallallaahu ʻalayhi wa sallam, attributed a lie to Allaah - Allaah forbids - He would have swiftly punished him, seizing him with a seizure of one Exalted in Might and Perfect in Ability for He is over all things competent and All-Wise. His wisdom entails not delaying the punishment of the liar who attributes a lie to Him. He aided His Messenger with miracles and signs , blessed him with victory, caused him to triumph over his enemies, and granted him power over them; this is the greatest testimony to the truthfulness of his message with which he was sent.”

Sixthly, he who lies is essentially imperfect, seeking to amend his imperfection by means of lying; verily, Allaah, The Exalted, is The Creator, The Self-Sufficient; He does not need to lie, nor does it befit Him. He is far exalted above such imperfection.

Allaah knows best.