I then found another corroborating report for the hadith: the hadith of Abu Dharr, may Allah be pleased with him, attributed to the Prophet, peace and blessings be upon him, with the wording:

“Indeed, that will be a light for you in the heavens and a light on earth.”

Al-Tabarani recorded it in al-Mu‘jam al-Kabir (1/82/2), through Ibrahim ibn Hisham ibn Yahya al-Ghassani, who said: My father narrated to me from my grandfather, from Abu Idris al-Khawlani, from Abu Dharr. However, this Ibrahim is weak; indeed, some accused him.

556 — “We would count the Messenger of Allah, peace and blessings be upon him, saying in one sitting: ‘My Lord, forgive me and accept my repentance. Indeed, You are the Ever-Accepting of Repentance, the All-Forgiving,’ one hundred times.”

Ahmad recorded it (2/21): Ibn Numayr narrated to us from Malik—that is, Ibn Mighwal—from Muhammad ibn Suqah, from Nafi‘, from Ibn ‘Umar; and he mentioned it.

I say: This is an authentic chain of transmission meeting the conditions of al-Bukhari and Muslim. However, the narrators differed in their reporting from Malik concerning the word “the All-Forgiving.” Ibn Numayr reported this word from him, and al-Muharibi corroborated him in al-Tirmidhi (2/254). But al-Muharibi differed in Ibn al-Sunni (364), where he said, “the Most Merciful,” in place of “the All-Forgiving.”

Abu Usamah likewise reported from Malik, in Abu Dawud (1516) and Ibn Majah (3814), saying, “the Most Merciful.” Ibn Majah paired al-Muharibi with Abu Usamah, though reports from al-Muharibi likewise differ concerning this word. Sufyan also reported from Malik, in Ibn Hibban (2459), saying the same. His narration is also found in al-Tirmidhi, but he did not give its wording, referring instead to al-Muharibi’s narration and saying, “Something similar in meaning.” Thus, I do not know whether this word in al-Tirmidhi’s narration from Sufyan is “the All-Forgiving” or “the Most Merciful.”

In any event, this is a serious discrepancy, and I am unable to give preference to either version: Ibn Numayr and al-Muharibi agreed on the first wording, while Abu Usamah and Sufyan agreed on the other.