Does a 90% correct prayer count for nothing?
The answer is NO:
‘Ammar b. Yasir said: I heard the apostle of Allah (ﷺ) say: A man
returns after performing his prayer while a tenth part of his prayer, or a
ninth part, or an eight part, or a seventh part, or a sixth part, or a
fifth part, or a third part, or half of it, is recorded for him.
Sunan Abi Dawud 796
The hadith you mentioned “Allah does not accept any prayer without tohoor” (tohoor translated as purification) and means ‘Wudu’ or ‘Tayammum’, so that is not related to the subject you are discussing.
The whispers of Satan
- Satan may whisper evil to you that your prayer is not accepted, so
you get disappointed and stop praying.
- Whispers are two types (*), light thoughts do not affect your concentration and understanding of what is being read, a prayer without those thoughts is better, but if they happen your prayer is still correct. The other type is thoughts which affect realization and understanding of the contents of the prayer and that will cause cutting parts of the reward as much as you miss of the prayer.
(*) Ibn Taymiyyah - Majmoo’ Fataawaa – 22 – (Do whispers cancel the prayer)
So the max would be losing all the reward, but yet that is not a reason to stop praying because then that person will not get rewarded but will not be punished for not performing the prayer so you can say he gets 0 but not minus.
Sunnah prayer supplements the deficiency of the obligatory
Prophet (ﷺ) said:
The first thing about which the people will be called to
account out of their actions on the Day of Judgment is prayer. Our
Lord, the Exalted, will say to the angels - though He knows better:
Look into the prayer of My servant and see whether he has offered it
perfectly or imperfectly. If it is perfect, that will be recorded
perfect. If it is defective, He will say: See there are some optional
prayers offered by My servant. If there are optional prayer to his
credit, He will say: Compensate the obligatory prayer by the optional
prayer for My servant. Then all the actions will be considered
Sunan Abi Dawud 864
Well perform wudu, well perform Rukou, and well perform Sujood (do not rush).
... The Prophet said, "When you get up for the prayer, perform the ablution properly and then face the Qibla and say Takbir (Allahu Akbar), and then recite of what you know of the Qur'an, and then bow, and remain in this state till you feel at rest in bowing, and then raise your head and stand straight; and then prostrate till you feel at rest in prostration, and then sit up till you feel at rest while sitting; and then prostrate again till you feel at rest in prostration; and then get up and stand straight, and do all this in all your prayers."
Sahih al-Bukhari 6667
Some (not all) of the things that prevent accepting prayer:
“Whoever drinks Al-khamr and gets drunk, his prayer will not be accepted
for forty days…"
Sunan Ibn Majah Vol. 4, Book 30, Hadith 3377
Prophet (ﷺ) who said, "He who goes to one who claims to tell about
matters of the Unseen and believes in him, his Salat (prayers) will
not be accepted for forty days."
Scholars explained acceptance as levels:
- Satisfaction, love and praise
- Reward but not as good as the first one
- Prevent punishment but no reward and this is when one prays but spend the prayer thinking about other things. (Ibn Al-Qayyim)