Bukhari

Hadith Commentary:
One rak‘ah (unit of prayer) is established from the Messenger of Allah (sallallahu alayhi wa sallam) himself; most likely, it was upon this very hadith that Amir Mu‘awiyah (radi Allahu anhu) acted. Even today, the majority of the Jama‘at Ahl al-Hadith act upon this hadith. Although performing witr as 7, 5, or 3 rak‘ahs is also permissible, the name "witr" refers specifically to the final single rak‘ah. From the response of Abdullah ibn ‘Abbas (radi Allahu anhu), it is apparent that he considered Mu‘awiyah (radi Allahu anhu) to be a jurist (faqih) and regarded his religious practice as authoritative. This also establishes a virtue (manqaba) of Mu‘awiyah (radi Allahu anhu), and this is in accordance with the chapter heading.

Hadith Commentary:
➊
These ahadith indicate the virtue of Muawiyah (radi Allahu anhu), that along with being distinguished by the honor of companionship (sahabiyyah), he was also a jurist (faqih), as is evident from the response of Ibn Abbas (radi Allahu anhu).
Those elders considered Amir Muawiyah a jurist and regarded his legal practice as authoritative.
Performing witr as a single rak‘ah is itself established from the Messenger of Allah (sallallahu alayhi wa sallam).
This was the practice of Muawiyah (radi Allahu anhu).
The claim of Allamah Ibn at-Teen that none of the jurists (fuqaha) held the view of witr being a single rak‘ah is a claim without evidence, as Hafiz Ibn Hajar (rahimahullah) has noted.
(Fath al-Bari: 7/132)

➋
There are numerous ahadith regarding Muawiyah (radi Allahu anhu) present in the books of hadith which indicate his virtue, but they were not in accordance with the conditions of Imam Bukhari (rahimahullah), hence they were not mentioned.
The Messenger of Allah (sallallahu alayhi wa sallam) summoned Muawiyah (radi Allahu anhu) for a certain task; it was found that he was eating. This happened two or three times, so the Prophet said:
“May Allah never fill his belly.”
(Sahih Muslim, al-Birr wa’s-Silah wa’l-Adab, Hadith: 6628(2604))
From this hadith, it is understood that it was a task which only Muawiyah (radi Allahu anhu) could perform; otherwise, the Messenger of Allah (sallallahu alayhi wa sallam) would have assigned it to someone else.
As for the words at the end of the hadith, they were not in the form of a curse (bad-du‘a), but rather an expression of affection.
In Sahih Muslim, this hadith is titled with these words:
(Bab: Whoever the Prophet (sallallahu alayhi wa sallam) cursed, reviled, or supplicated against, though he was not deserving of it, it would be a purification, reward, and mercy for him)
“Whoever the Messenger of Allah (sallallahu alayhi wa sallam) cursed, or spoke ill of, or supplicated against, though he was not deserving of it, then those prophetic words would be a means of purification, reward, and mercy for that person.”
In any case, there is no reason for any change in the title of Imam Bukhari (rahimahullah), as some commentators have tried to suggest.
And Allah knows best.

Ata bin Abi Rabah rahimahullah narrates:
«أن معاوية أوتر بركعة، فأنكر ذلك عليه، فسئل ابن عباس، فقال: أصاب السنّة .»
“Sayyiduna Muawiyah radi Allahu anhu performed one rak‘ah of witr. He was objected to regarding this matter. Sayyiduna Ibn Abbas radi Allahu anhuma was asked about it, and he said: He has acted upon the Sunnah.” [مصنف ابن ابي شيبة: 291/2، وسنده صحيحٌ]
One rak‘ah witr is Sunnah; it is also a sign of a jurist (faqih) that he affirms and performs one rak‘ah witr.
Sayyiduna Muawiyah and Sayyiduna Ibn Abbas radi Allahu anhum—two eminent Companions—affirmed and performed one rak‘ah witr.

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Practice of Other Companions
Abu Mujliz narrates that I asked Sayyiduna Ibn Umar radi Allahu anhuma about witr and said: If I am traveling, what should I do? He radi Allahu anhu replied:
«ركعة من آخر الليل .»
“Pray one rak‘ah in the last part of the night.” [مصنف ابن ابي شيبة: 301/2، وسنده صحيحٌ]
Abdur Rahman Taymi narrates: I thought that tonight no one would surpass me in standing for the night prayer (qiyam al-layl). I got up and heard the sound of someone behind me. It was Sayyiduna Uthman bin Affan radi Allahu anhu. I moved aside. He radi Allahu anhu stepped forward, began reciting the Noble Qur’an and completed it, then bowed and prostrated. I thought: The Shaykh has forgotten. When he radi Allahu anhu finished the prayer, I said: O Commander of the Faithful! You performed only one rak‘ah witr. He radi Allahu anhu replied: Yes. This is my witr.” [شرح معاني الآثار للطحاوي: 294/1، سنن الدارقطني: 34/2، ح، 1656-1658، و سنده حسنٌ]
Abdullah bin Muslima narrates that Sayyiduna Sa‘d bin Abi Waqqas radi Allahu anhu led us in the ‘Isha prayer, then in a corner of the mosque performed one rak‘ah. I followed him and asked: O Abu Ishaq! What is this rak‘ah? He replied: Witr, anaam ‘alayhi. “This is witr, after which I will sleep.” Amr bin Murrah says: I informed Mus‘ab bin Sa‘d of this, and he said: Sayyiduna Sa‘d radi Allahu anhu used to perform one rak‘ah witr. [شرح معاني الآثار للطحاوي: 295/1، و سنده حسنٌ]

One, three, five, and seven witr according to the Hanafis
It is permissible to perform one, three, five, or seven rak‘ahs of witr. ➌ Mawlana Anwar Shah Kashmiri Deobandi writes:
«نعم، ثابت عن بعض الصحابة بلا ريب .»
“Yes, from some of the Companions, without any doubt, performing one rak‘ah witr is established.” [العرف الشذي للكشميري: 12/2]