Imam Hussain (A.S) asked God to illuminate his face.

When people of the tribe of Asad came after three days to bury the martyrs, they were astonished to find a corpse which was shining with heavenly light and enveloped in heavenly perfume.
This is the story of a man who was one of the sincerest adherents of the Ahl al-Bayt (A.S).
Jawn bin Huwai, of Ethiopia, was probably a convert from Christianity as his name suggests. He was a slave of Abu Dharr Al-Ghifari (RA), the famous companion of the Prophet (PBUH). After the death of Abu Dharr (RA), he attached himself to the Ahlul Bayt who were looking after him.
He accompanied Imam Hussain (A.S) to Karbala, and although by that time an old man, he tried to go to the battlefield to fight. Imam Hussain (A.S) at first refused; but Jawn persisted and, at last, Imam (AS) allowed him to go to the battlefield.
Imam Hussain (A.S) was so moved by Jawn’s martyrdom, in the same manner he was moved by the martyrdom of his son, Ali Akbar (AS). When he fell down, Imam Hussain (A.S) went to his corpse, sat by him, held John’s head—covered in blood—on his lap. Then everyone saw Imam Hussain (A.S) holding John’s face to his own face.

Source: en.shafaqna.com