Luke 24:5

It is worthy of remark, that none of the disciples or friends of Christ, were so much astonished and struck at the many apparitions of angels, &c. as to be cast down to the ground, as the guards and his enemies were, but only through respect and reverential fear looked down upon the ground. Nor even did any of them fall down prostrate to adore our Saviour, when he appeared to them; because Christ was not now to be sought in the earth, among the dead, but was risen, and was to be looked for from heaven. Hence is derived the Catholic custom of praying in Pascal time, and on all Sundays, &c. not on the knee, but with the body respectfully bent, and bowing down their countenance towards the ground. (Ven. Bede)