What is the origin of closing one's eyes during prayer?

I have been a Christian since my youth, but I have never received a satisfactory answer to this question. **Why must a Christian close their eyes during prayer?** In large services in the west, (I am not familiar with other traditions) believers bow their heads and close their eyes while being led in prayer, often speaking internally to God or self reflecting. Bowing one's head, or prostrating themselves is itself a form of worship. Personally, in moments of personal prayer I do not close my eyes because I find it more distracting. But in my family and church I grew up in this would be seen as very offensive if someone might walk in and see me praying with my eyes open. I'm mostly interested in the scriptural basis for closing one's eyes while praying. Not so much the historical reason for the tradition developing, although I recognize it might be difficult to separate the two.