Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
The hadith “The best remembrance is Lā ilāha illā Allāh” is established and hasan (sound). It is also reported that “Lā ilāha illā Allāh” is the best statement said by the Prophet, peace and blessings be upon him, and the prophets before him. As for the addition, “and it is the Greatest Name of Allah,” it is not part of the hadith as a statement attributed directly to the Prophet, peace and blessings be upon him. Rather, it is an expression mentioned by some scholars as an opinion or inference, and it was subsequently transmitted in some books alongside the citation of the hadith, which may give the impression that it was part of the Prophet’s statement.
Al-Tirmidhi (3383), Ibn Majah (3800), Ibn Hibban (846), al-Hakim (1834), and al-Nasa’i in al-Sunan al-Kubra (10599) narrated from Jabir ibn Abdullah, may Allah be pleased with them both, that the Prophet, peace and blessings be upon him, said:
“The best remembrance is Lā ilāha illā Allāh, and the best supplication is al-hamdulillāh.”
Al-Tirmidhi graded it hasan, and al-Albani graded it hasan in Sahih al-Tirmidhi.
Al-Tirmidhi (3585) also narrated from Abdullah ibn Amr, may Allah be pleased with him and his father, that the Prophet, peace and blessings be upon him, said:
“The best supplication is the supplication of the Day of Arafah, and the best that I and the prophets before me have said is: Lā ilāha illā Allāh wahdahu lā sharīka lah, lahu al-mulku wa lahu al-hamd, wa huwa ‘alā kulli shay’in qadīr.”
Al-Albani graded it hasan in Sahih al-Tirmidhi.
Imam Malik narrated in al-Muwatta’ (726), and al-Bayhaqi in al-Sunan (8391), from Talhah ibn Ubaydullah ibn Kurayz that the Prophet, peace and blessings be upon him, said:
“The best supplication is the supplication of the Day of Arafah, and the best that I and the prophets before me have said is: Lā ilāha illā Allāh wahdahu lā sharīka lah.”
These texts demonstrate the tremendous virtue and status of the statement of tawhid.
The phrase that has caused confusion is:
“It is the statement of tawhid and sincerity, and it is the Greatest Name of Allah.”
This is not part of the Prophet’s statement in the aforementioned hadith.
Rather, it was stated by some scholars.
Al-Qadi Iyad, may Allah have mercy on him, transmitted in Ikmal al-Mu‘lim (8/192), after mentioning the virtue of saying the tahlil:
“It has been said that it is the Greatest Name of Allah, and it is the statement of sincerity.”
Al-Hafiz Ibn Hajar, may Allah have mercy on him, then quoted this statement in Fath al-Bari (11/207), followed by the citation of the hadith “The best remembrance is Lā ilāha illā Allāh.”
Later, al-Badr al-Tamam included wording that combined the hadith with this statement. Then al-San‘ani, may Allah have mercy on him, transmitted the statement in Subul al-Salam. As a result, the context may give the impression that the sentence “and it is the Greatest Name of Allah” is part of the hadith attributed to the Prophet.
In reality, it is not.
The reason is that some authors combined the wording of the hadith with the statements of scholars commenting on it, without clearly separating them in certain places.
Consequently, a researcher may read the text in some editions or electronic copies and assume that everything appearing within quotation marks is part of the Prophet’s statement, even though the final portion is not part of the hadith.
Therefore, citing the hadith in the hadith sources does not establish that the Prophet, peace and blessings be upon him, said:
“And it is the Greatest Name of Allah.”
What is authentically established from him is the virtue of Lā ilāha illā Allāh, that it is the best remembrance, and that it is among the best things he and the prophets before him said in the hadith concerning the virtue of the supplication of the Day of Arafah.
This is a separate question from whether the phrase is authentically established in the hadith.
Scholars have differed concerning the identification of the Greatest Name of Allah. Several hadiths have been reported concerning it, and scholars have derived different views from them.
As for attributing to the Prophet, peace and blessings be upon him, in this hadith the statement that “Lā ilāha illā Allāh” is the Greatest Name of Allah, that attribution is not authentic.
Scholars have previously discussed the matter of the Greatest Name of Allah and differed concerning its identification. Therefore, it is not correct to assert that this hadith indicates that the Prophet explicitly identified it with this phrase.
The hadith “The best remembrance is Lā ilāha illā Allāh” is hasan, and it is also established that “Lā ilāha illā Allāh” is among the best things said by the Prophet, peace and blessings be upon him, and the prophets before him. As for the statement “and it is the Greatest Name of Allah,” it is not part of the hadith attributed to the Prophet. Rather, it is a statement of some scholars that was transmitted in books of commentary and other works, and in some places it became connected to the wording of the hadith in a way that may give the impression that it was part of it.
Therefore, it is not correct to use this hadith as evidence that the Prophet, peace and blessings be upon him, explicitly stated that “Lā ilāha illā Allāh” is the Greatest Name of Allah.
And Allah knows best.