Assalamu alaykum. Abou Hourayra (may Allah be pleased with him) narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said, "Do not insult the devil, and seek protection of Allah against his evil." (Reported by Al-Daylami and authenticated by Shaykh Al-Albaani in Silsila Saheehah No. 2422) My question is the following: Is insulting and speaking bad about the devil some bigger sin or it is disliked ?

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 ‘alayhi wa sallam, is His slave and Messenger.

The hadeeth that you mentioned, although it was classified by Shaykh al-Albaani as authentic, was graded weak by some prominent scholars of hadeeth, such as ad-Daaraqutni in his book entitled al-‘Ilal.

In any case, the hadeeth does not imply that insulting the devil is forbidden, let alone being among the major sins. We are not aware of any scholar who said that insulting the devil is forbidden in principle.

The hadeeth advises to stop insulting the devil because there is no benefit in doing so, lest a Muslim would be accustomed to insulting and saying obscene words, and because the most effective way to avoid the plots of the devil is to seek refuge in Allaah from him; Allaah says (what means): {And if there comes to you from Satan an evil suggestion, then seek refuge in Allaah. Indeed, He is the Hearing, the Knowing.} [Quran 41:36]

So it is more appropriate to seek refuge in Allaah from him than being preoccupied by insulting him.

Faydh al-Qadeer reads, “Do not insult the devil, as insulting him does not repel his harm from you and does not protect you from his hostility in any way; rather, seek refuge with Allaah from his evil, as it is Allaah Who controls him and Who repels his plots from whomever He wills of His slaves.”

Since it is allowed to curse Satan, then insulting him is not forbidden with greater reason. The fatwa of the Standing Committee reads:

"With regard to cursing the devil, Allaah cursed him in His book many times, when he was arrogant and did not comply with the order of Allaah to prostrate to Aadam (Adam) when Allaah created the latter; of course, this was a prostration of honor and reverence. Allaah described the devil as being accursed, and he is expelled from the Mercy of Allaah and His Paradise on the Day of Judgment.

Allaah says (what means): {They call upon instead of Him none but female [deities], and they [actually] call upon none but a rebellious Satan. Whom Allaah has cursed. For he had said: I will surely take from among Your servants a specific portion.} [Quran 4:117-118]

Allaah also says (what means): {[Allaah] said: Then get out of it, for indeed, you are expelled. And indeed, upon you is the curse until the Day of Recompense.} [Quran 15:34-35]

Also, the Prophet cursed him in prayer. Abu ad-Darda' narrated: "Allaah’s Messenger, sallallaahu ‘alayhi wa sallam, stood up (to pray) and we heard him say, 'I seek refuge in Allaah from you.' Then he said, 'I curse you with Allaah's curse,' three times, then he stretched out his hand as though he was taking hold of something. When he finished the prayer, we said, 'O Messenger of Allaah, we heard you say something during the prayer that we have not heard you say before, and we saw you stretch out your hand.' He replied, 'Allaah's enemy Iblees (the devil) came with a flame of fire to put it in my face, so I said three times, 'I seek refuge in Allaah from you.' Then I said three times, 'I curse you with Allaah's full curse.' But he did not retreat on any one of these three occasions. Thereafter, I meant to seize him. I swear by Allaah, had it not been for the supplication of my brother Sulayman [Prophet Solomon – who made a supplication saying, {My Lord, forgive me and grant me a kingdom such as will not belong to anyone after me. Indeed, You are the Bestower.} (Quran 38:35)], then he would have been tied up, and made an object of sport for the children of Madeenah to play with.' [Muslim]"

Therefore, it is permissible for a person to curse the devil if the latter wanted to harm the person or tried to prevent him and whisper to him in order to repel him from the obedience of Allaah. However, a person should not refrain from seeking refuge in Allaah from him, mention Allaah much, and say, 'Bismillaah' (In the name of Allaah) and other mention of Allaah and legislated supplications so that a Muslim would be protected by Allaah from the evil of the devil and as a way of being pursuant to the previous verses and ahaadeeth. A person should not make cursing the devil as his habit without a reason."

Allaah Knows best.