The most authentic version of this Istighfar
First of all the most authentic version of this du'a is:
“My father narrated to me, from my grandfather, that he heard the Prophet (ﷺ) say: ‘Whoever says: “I seek forgiveness from Allah, the Magnificent, whom there is none worthy of worship but Him, the Living, Al-Qayyum, and I repent to him,”
(Astaghfiru llāh al-`Aẓīm alladhī lā ilāha illā huwal-Ḥayyul-Qayyūmu wa atūbu ilaih
أستغفر الله العظيم الذي لا إله إلا هوالحي القيوم وأتوب إليه)
then Allah will forgive him, even if he fled from battle.’” (Jami' at-Timridhi)
This was qualified as hassan on sunnah.com and as sahih by al-Albani (however my impression hassn and sahih in al-Albani's terminology are refereed to as sahih).
Nevertheless in fatwa islamqa #277420 this and other ahadith like the one recommending to say it three times -which is da'if- are discussed and they say that it is rather unlikely that the word and Allah's attribute:
the Magnificent "العظيم" (al-'Adhim or al-'Azim)
really appeared in the original of Jami' at-Tirmdihi as in the manuscripts which Ahmad Shakir had at hands while compiling his unfinished edition only one dated to 1242 a.H. had this addition and it appears neither in imam al-Bukhari's works -and imam at-Tirmidhi quoted him as the teacher from which he reported this- who compiled it in his Tareekh, nor in Sunan abi Dawod (Same narrator chain as al-Bukhari) nor ibn Sa'ad's at-Tabaqaat, nor Musannaf ibn abi Shaybah (version of three times on the authority of Mu'ad ibn Jabal and on the authorithy of ibn Mas'ud and version of five times on the authorithy of abu Sa'id al-Khudri) while imam al-Hakim from Nishapure quoted it twice in his al-Mustadrak once in the same manner as al-Bukhari and once in the version quoted above as appears on sunnah.com in Jami' at-Tirmidhi. Al-Albani further added the following information: Imam as-Suyuti didn't mention the word "Magnificient" in his al-Jami'a l-Kabir, it only appears in al-Hakim's al-Mustadrak therefore he considers it as doubtful that this word was part of the original version of imam at-Tirmdihi's hadith collection. In this version imam a-Dhahabi commented that al-Hakim used a narrator which imam al-Bukhari didn't report from in his sahih so the statement of al-Hakim that this version on the authority of ibn Masu'd is on the conditions of both al-Bukhari and Muslim isn't 100% correct. While the other version of al-Mustadrak has an issue in the narrator chain (one of the narrators has apparently reported three different statements).
And therefore imam an-Nawawi quoted the most acceptable version his Riyadh as-Saliheen:
Ibn Mas'ud (May Allah be pleased with him) said:
The Messenger of Allah (ﷺ) said, "He who says: 'Astaghfir ullah-alladhi la ilaha illa Huwal-Haiyul-Qayyumu, wa atubu ilaihi (I seek the forgiveness of Allah, there is no true god except Allah, the Ever-Living, the Self- Subsisting, and I turn to Him in repentance),' his sins will be forgiven even if he should have run away from the battlefield (while he was engaged in fighting for the Cause of Allah)."
[Abu Dawud, At-Tirmidhi and Al-Hakim (on conditions of Al-Bukhari and Muslim for accepting Hadith)].
What that this means? This means again no version giving a recomendation about an amount of repetitions reaches a level of good (hassan) and further the most authentic version is rather:
أستغفر الله الذي لا إله إلا هوالحي القيوم وأتوب إليه
The version of ibn abi Shaybah
I've shared the versions on the three times repetition of the hadith on the authority of Mua'dh ibn Jabal and 'Abdullah ibn Mas'ud, and the five times repetition on the authorithy of abu Sa'id al-Khudri in which Allah's attribute was not quoted above in both cases it is a statement of a Sahabi -but can be regarded as hadith marfo' as the sahabi can hardly come with this on his own- (it has the same narrator chain which may mean that some narrator had a doubt...).
Ibn abi Shaybah also quoted a version of a five times repetition on the authority of abu Sa'id al-Khudri in which this attribute was mentioned:
The translation is based on the da'if version from imam at-Tirmidhi's book which is linked above!
29447 - حدثنا عفان، حدثنا بكير بن أبي السميط، حدثنا منصور بن زاذان، عن أبي الصديق الناجي، عن أبي سعيد الخدري، قال:
" من قال: أستغفر الله العظيم الذي لا إله إلا هو الحي القيوم، وأتوب إليه خمس مرات غفر له، وإن كان عليه مثل زبد البحر " (Source)
'Affan informed us, we were informed by Bukayr ibn abi as-Sameet, from Mansur ibn Zaadaan, from abu as-Siddeeq an-naji, from abu Sa'id al-Khudri, who said:
Whoever says: ‘I seek forgiveness from Allah, [the Magnificent] the One whom there is none worthy of worship except for Him, the Living, the Sustainer, and I repent to Him (Astaghfirullāha [al-`Aẓim] alladhi lā ilāha illā huw, al-Ḥayyul-Qayyūm, wa atūbu ilaihi)’ five times, Allah shall forgive him his sins if they were like the foam of the sea.
As for the hadith transmitters:
- 'Affan: is 'Affan ibn Muslim as-Saffar عفان بن مسلم الصفار the hafidh of al-'Iraq who was born around 134 a.H. according imam a-Dhahabi, he died 220 a.H.. A trustworthy scholar and teacher of abu Bakr ibn abi Shaybah and al-Bukhari and all other authors of the six books narrated from him via intermediate.
- Bukayr ibn abi as-Sameet بكير بن أبي السميط المسمعي a Basri scholar, he is regarded as trustworthy by many scholars however when we do a deep check those who are regarded less lenient have some remarks: Ibn Hebban even if he added him in his book at-Thiqaat says one can't rely on his narration if he was the lonely narrator as he had memory issues, ibn Ma'yn and abu Hathim ar-Razi said respectively: Salih and Laa ba'sa bihi (which would qualify him as of a mid or low level of trust) and imam an-Nasa-i only reported one single narration from him in his Sunan. This should make clear that this narrator might already be an issue in this chain.
- Mansur ibn Zaadaan منصور بن زاذان (from Waasit-buried there-) died 131 a.H. is one of the later Tabi'yn and was a companion of al-Hassan al-Basri. He is regarded as trustworthy by imams like Ahamd ibn Hanbal, abu Hathim ar-Razi, ibn Sa'ad, ibn Ma'yn, an-Nasa-i, ad-Daraqotni, a-Dhahabi and ibn Hajar...
- Abu as-Siddeeq an-Naaji أَبُو الصديق الناجي his name was Bakr abu 'Amr , while some said abu Qays. He is a tabi'y from al-Basra who died 108 a.H. according ibn Hebban. He reported from three known sahabah: abu Sa'id al.Khudri, 'Abdullah ibn 'Umar and 'Aishah. Ibn May'n, an-Nasa-i and abu Zura'ah regarded him as trustworthy as stated in al-Mizzi's Tahdeed al-Kamal (in biography #751, he was mentioned several times in the book). However ibn Sa'ad stated that some people were discussing about his qualities in his at-Tabaqaat.
As you see the narrator chain of al-Musannaf isn't one which has big issues. Nevertheless and addition of a word or an amount of repetitions would need a certain back up.