Muhammad ibn Muhammad ibn al-Athir al-Jazari, brother of al-Mubarak, said: He was an imam, a hafiz, and he had excellent works. (1)

Abu Ja'far ibn al-Zubayr said: Al-Tirmidhi possessed, in the various branches of the craft of hadith, what no one else shared with him. (2)

Ibn al-'Imad al-Hanbali said: He excelled over his peers, a sign of memory and precision. (3)

Ali al-Qari said: The imam, the proof, the unique one, the trustworthy, the precise hafiz. (4)

This imam, whose fame has spread across the horizons, whose book has spread throughout the lands of Islam, and whom the scholars of his time and those who came after them among the people of knowledge have praised—it does not harm him that Abu Muhammad ibn Hazm said of him in his book "al-Isal" (5): that he is unknown, and it is not known who he is. For this very statement proclaims against its speaker his limited vision, his scant acquaintance, and the paucity of his knowledge in this field.

--------------------------------------------
(1) "al-Kamil fi al-Tarikh" 7/460, under the events of the year 279 AH.
(2) Quoted from him by al-Suyuti in "Qut al-Mughtadhi" p. 6.
(3) "Shadharat al-Dhahab" 2/174-175.
(4) "Mirqat al-Mafatih Sharh Mishkat al-Masabih" 1/21.
(5) Al-Dhahabi said in "Tadhkirat al-Huffaz" 2/1147 about this book: It is a large book on the jurisprudence of hadith, and he named it: "al-Isal ila Fahm Kitab al-Khisal al-Jami'ah li-Jumal Sharai' al-Islam wa al-Halal wa al-Haram wa al-Sunnah wa al-Ijma'."