Authenticity of the hadith
First of all there are at least three main variations of the hadith itself:
The one of imam at-Tirmidhi in his Jami':
“Whoever says in the morning: ‘O Allah we have reached morning, calling You to witness, and calling the carriers of Your Throne to witness, and Your angels, and all of Your creation, that You are Allah, none has the right to be worshipped but You, Alone, without partner, and that Muhammad (ﷺ) is Your slave and Your Messenger. (Allāhumma aṣbaḥnā nush-hiduka wa nush-hidu ḥamalata 'arshika wa malā’ikataka wa jamī'a khalqika bi-annaka Allāh, lā ilāha illā anta, waḥdaka lā sharīka laka, wa anna Muḥammadan 'abduka wa rasūluk)’ Allah will forgive him for whatever he does that day, and if he says it in the evening, Allah will forgive him for whatever sin he commits that night.”
Imam at-Timridhi commented that this is ghareeb غريب.
The one which was compiled by imam abu Dawod in his Sunan speaking of saying the du'a four times and a similar one in al-Adab al-Mufrad of imam al-Bukhari only pointing at saying it four times.
So far all the narration are reported on the authority of Anas.
And further a version in which neither morning nor evening is mentioned which imam al-Hakim has compiled:
In the following I'll translate from Arabic language as tehse translations are of my own take them carefully!
حدثنا أبو العباس محمد بن يعقوب، ثنا أبو عبد الله أحمد بن يحيى الحجري، ثنا زيد بن الحباب، ثنا حميد بن مهران، ثنا عطاء، عن أبي هريرة رضي الله عنه، قال: حدثنا سلمان الفارسي قال: قال رسول الله صلى الله عليه وسلم: «من قال اللهم إني أشهدك وأشهد ملائكتك وحملة عرشك، وأشهد من في السماوات ومن في الأرض، أنك أنت الله لا إله إلا أنت وحدك لا شريك لك، وأشهد أن محمدا عبدك ورسولك، من قالها مرة أعتق الله ثلثه من النار، ومن قالها مرتين أعتق الله ثلثيه من النار، ومن قالها ثلاثا أعتق الله كله من النار» هذا حديث صحيح الإسناد، ولم يخرجاه " (Source)
We were informed by Abu al-Abbas Muhammad ibn Yaqoub, from Abu Abdullah Ahmad ibn Yahya al-Hajri, from Zaid ibn al-Habbab, From Hamid ibn Mahran, From 'Ata who reported on the authority of abu Hurairah, may Allah be pleased with him, who said: Salman al-Farisi told us, he said: The Messenger of Allah, Allah's prayers and blessing be upon him, said: Who said,
“Oh Allah, I bear witness to you and your angels and bearers of your throne, and I bear witness who is in the heavens and on the earth, that you are Allah, there is no deity but you alone, without partner, and I bear witness that Muhammad is your servant and your messenger." two-thirds of it from the Fire, and whoever says it three times, Allah will free all of them from the Fire.
This narration has a sane narrator chain, but they did not compile it.
As for the authenticity:
- The best one could say about the hadith version from imam at-Tirmidhi's Jami' is that it is regarded as hassan, the most critical narrator here is Baqiyah ibn al-Waleed which ibn Hajar considered as Sadooq in Nata'ij al-Afkaar نتائج الأفكار and ibn Hebban mentioned in his at-Theqaat الثقات.
- As for the version from abu Dawod it is rather da'if.
- Al-Haythami said in his Majma' a-Zawa'id مجمع الزوائد that at-Tabarani complied a version of the hadith with two different narrator chain in one of them was a person al-Haythami didn't know, but the rest of the narrators were narrators of the Sahih's. As for the version in al-Mustadrak imam a-Dhahabi confirmed al-Hakim's qualification of his version being Sahih and al-Albani accepted it however his statement shows that he would regard it rather hassan as he said that the narrations reported on the authority of Anas supports the authenticity of the the hadith:
أخرجه الحاكم (1 / 523) من طريق حميد بن مهران حدثنا عطاء عن أبي هريرة رضي الله عنه قال: حدثنا سلمان الفارسي قال: قال رسول الله صلى الله عليه وسلم: فذكره، وقال: " صحيح الإسناد "، ووافقه الذهبي وهو كما قالا. وله شاهد من حديث أنس مرفوعا نحوه مقيدا بالصباح والمساء، وسنده ضعيف كما بينته في " سلسلة الأحاديث الضعيفة " رقم (1041)
It was compiled by Al-Hakim (1/523) via Hamid ibn Mahran, 'Ata narrated to us on the authority of Abu Hurairah, may Allah be pleased with him, he said: Salman Al-Farisi told us, he said: The Messenger of Allah, Allah's prayers and blessing be upon him: And he mentioned it, and said: “The chain of transmission is authentic (Sahih).” Al-Dhahabi agreed with him, and it is as both said. And it has a witness from the hadith of Anas, with a chain of transmission related to it, tied to morning and evening, and its chain of transmission is weak, as I explained it in “Silsilat al-Ahadith al-Da’eefah” No. 1041. (Source)
So the hadith is only sahih without mentioning a specific timing for this dhikr.
The term ghareeb
ghareeb in Arabic means strange.
As for the terminology of hadith scholars this term is less a term for qualifying a hadith from the perspective of authenticity then from the perspective on fame and tawatur. As rather early in the development of hadith terminology scholars agreed upon terms of ahaad hadith, ghareeb actually is used if a hadith is only reported by one narrator in each or at least any of the levels of a narrator chain.
As for the terminology of imam at-Tirmidhi it is a bit different. And he gave us a hint about his terminology with examples in his book al-'ilal as-Sagheer كتاب العلل الصغير in a long explanation (much longer than that for hassan which are only two sentences) before discussing the topic of strangeness of a narrator chain:
وما ذكرنا في هذا الكتاب حديث غريب فإن أهل الحديث يستغربون الحديث لمعان رب حديث يكون غريبا لا يروى إلا من وجه واحد مثل ما حدث حماد بن سلمة عن أبي العشراء عن أبيه قال قلت يا رسول الله أما تكون الذكاة إلا في الحلق واللبة فقال لو طعنت في فخذها أجزأ عنك فهذا حديث تفرد به حماد بن سلمة عن أبي العشراء ولا يعرف لأبي العشراء عن أبيه إلا هذا الحديث وإن كان هذا الحديث مشهورا عند أهل العلم وإنما اشتهر من حديث حماد بن سلمة لا يعرف إلا من حديثه فيشتهر الحديث لكثرة من روى عنه ... (Source)
And what we mentioned in this book is a strange hadith, for the people of hadith are surprised by the meaning of the hadith, perhaps a strange hadith that is narrated only from one aspect, like what Hammad ibn Salamah narrated on the authority of Abu al-'Ashra' on the authority of his father. He said: I said, O Messenger of God, is the sacrifice only in the throat and core? He said, “If you were stabbed in the thigh, it will be sufficient for you. This is a hadith that was unique to Hammad ibn Salamah on the authority of Abu al-'Ashra'. It is not known to Abu al-'Ashra' on the authority of his father except this hadith, although this hadith is well-known among the scholars, but it is more famous than a hadith Hammad ibn Salama is only known from his hadith, so the hadith is famous due to the large number of people who narrated from him...
Let me summarize it in four points (at least one of them is already presented above), ghareeb in the terminology of at-tirmidhi is when:
- If it was only narrated in a single version from a single person. (ghareeb as it is commonly known).
- If a hadith is known only via a certain scholar and from his level on it became famous.
- If a hadith is strange due to an addition which appears in it which can only be accepted if the narrator who is known for his good memory.
- If a hadith is known from many narrators and narrator chains, but a specific narrator chain seem strange.
Be aware that there's a three volume book only discussing the term "ghareeb" in the terminology of imam at-Tirmidhi -as I've read ion the site of the contemporary scholar ibn Amin -see here-- so these statements above are a very very superficial information.
Another reference fatwa islamweb #34520.
Additional sources and references provided by OP: