Why he would turn pale during wudu

حدثني عبد الرحمان بن حفص القرشي قال: كان علي بن حسين إذا توضأ اصفر، فيقول له أهله: ما هذا الذي يعتادك عند الوضوء؟ فيقول: تدرون بين يدي من أريد أن أقوم؟

حلية الأولياء

It is said about Ali bin Husayn رحمة الله عليه when he would perform ablution he would turn pale. It was asked from him: ‘what happens to you when you perform ablution?’ He said: Do you know who It is who I intended to stand in front of?