At the time of worship, such a fear used to overcome Imam Zainul Abideen (a.s) that the color of his face used to become yellow (Pale). This condition endured from the beginning till the end. At the time of ritual ablution (wuzu) the same condition occurred. Once someone asked the reason for it. Imam (a.s) replied, “At this moment I am standing before a glorious and a divine Emperor Who is the Creator of all the worlds, in Whose hands is the reward and punishment of every creature. What is so surprising that my condition is such due to His fear?”
Once he went to perform the Hajj pilgrimage. When he reached the point where pilgrims put on the ritual dress he was about to recite the Thalbiya (Labbaik) and put on the dress when all of a sudden the color of his face underwent a change and the body began to shake terribly. At last, he could not even utter Labbaik. People asked him why he didn’t recite the thalbiya. He said, “I was afraid to say Labbaik (here I am at your service) lest Allah may reply: La-Labbaik (I am not for you).” Saying this he wept so much that he lost consciousness. He performed all the rituals in this fearful condition. Imam Zainul Abideen (a.s) used to recite altogether one thousand rakats (units) of prayers in twenty-four hours, and in every prayer he used to tremble severely.
Reference :Akhlaq al-A’imma, Morals & Manners of the Holy Imams