Testimony by Al-Hakim al-Naysaburi:

Imam Al-Hakim said: I heard ‘Umar ibn ‘Alaq say, “When Imam al-Bukhari passed away, he did not leave behind in Khurasan anyone like Abu ‘Isa al-Tirmidhi in knowledge, memorization, piety, and asceticism. Al-Tirmidhi wept so much out of the fear of Allah that he became blind and remained blind for many years.” [Siyar A‘lam al-Nubala’ 13/273, Tadhkirat al-Huffaz 2/634, Tahdhib al-Tahdhib 9/389]

The Hadith master of Khurasan, Imam Abu Ahmad al-Hakim al-Naysaburi (d. 378 AH), narrated from one of his teachers that when Imam Muhammad ibn Isma'il (al-Bukhari) passed away, he did not leave behind among his students anyone superior to Imam al-Tirmidhi in knowledge, memorization, asceticism, and piety. Al-Tirmidhi would weep so much out of the fear of Allah that he eventually became blind.