Imam Sajjad (a.s) in words of Sunni scholars 6

Imam Sajjad (a.s) in words of Sunni scholars
Ahamd al-Ya’qubi
Ali ibn al-Husayn (as) was the best of the people and the most devout of them, so he was called Zayn al-Abidin. He has also been called the owner of the calluses (Zul-Thafnat), because there were traces of prostration on his forehead.
Tarikh al‑Yaqubi,vol3,p46