Assalaamu alaykum wa rahmatullaahi wa barakaatuh. Shaykhs, is there a way to know if an evil thought that came to one's mind is from the evil of one's own self or from the devil? And whenever an evil thought comes to mind, must we attribute it to the devil and seek refuge with Allah, the Exalted, or must we do this and also attribute it to our own evil self and also seek Allah's protection from the evil of our own self? And if you can, please mention who is the biggest enemy to us; the devil or our own evil self. May Allah reward you, respected Shaykhs.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

The apparent meaning of Islamic texts is that evil thoughts come from one’s own self and from the devil.

Indeed, the Prophet combined the evil of the self and the evil of the devil in his authentic supplication of the morning and the evening and before going to sleep.

The supplication reads, “Allahumma ‘Aalim al-ghaybi wash-shahaadah, Faatir as-samaawaati wal-ardh, Rabba kulli shay-in wa Maleekahu, ashhadu an la ilaaha illa Anta, a‘oothu bika min sharri nafsee wa min sharri ash-shaytaani wa shirkihi.” (O Allah, Knower of the unseen and the seen, Creator of the heavens and the Earth, Lord and Sovereign of all things, I bear witness that none has the right to be worshipped except You. I take refuge in You from the evil of my soul and from the evil and shirk of the devil.) [At-Tirmithi: good-authentic; Abu Daawood; Ahmad; Al-Albaani: authentic]

In any case, you should seek refuge in Allah from the evil of both of them.

Among what was reported from the Prophet in Khutbat-ul-Haajah (i.e. the opening speech, with which the imaam usually starts the Friday sermon or any other sermon) is: “…and we seek refuge in Allah from the evil of our own selves and the evil of our sinful deeds…” [Ahmad, Abu Daawood, At-Tirmithi, An-Nasaa’i, and Ibn Maajah - Al-Albaani graded it Saheeh (sound)]

As for the supreme enemy, it is apparent – and Allah knows best – that the devil poses the most intense enmity and most serious danger because he is pure evil, and the Quran is replete with warnings against him and that he is a clear enemy; Allah says (what means): {Indeed, Satan is an enemy to you; so take him as an enemy. He only invites his party to be among the companions of the Blaze.} [Quran 35:6]

As for the soul, it is one of the soldiers of the devil; he orders it and it obeys him, and perhaps it disobeys him, by the grace of Allah, and so it is a good soul.

Ibn Al-Qayyim said in Kitaab Al-Fawaa’id:

“We have already mentioned that the soul is like a grinding stone that turns with whatever is thrown into it. If you throw grain into it, it will turn with it, and if you throw glass, stones, or dung into it, it will turn with it. Allah is the Disposer of that grinding stone, its Owner, and its Controller, and He appointed an angel to it who throws into it what benefits it, and it turns with it, and a devil who throws into it what harms it, and it turns with it. So the angel overcomes it once, and the devil overcomes it once. The grain that the angel throws into it is a promise of goodness and reward, whereas the grain that the devil throws into it is a promise of evil and a denial of that promise of goodness, and the grind is according to the grain. The one who has a harmful grain will not be able to throw it into the grinding stone unless he finds one that is empty of grain and its controller has abandoned it and rejected it; in which case, he (the devil) will cast whatever he has into it.”

There is a hadeeth that reads, “The worst of your enemies is your soul that is within you,” but this hadeeth is fabricated, as Al-Albaani stated in As-Silsilah Adh-Dha‘eefah.

Allah knows best.