Midrasz do Samuela I 6:12

(Fol. 24b) (I Sam. 6, 12) (Vayisharna) And the cows went straight forward. What does the expression Vayisharna mean? Said R. Jochanan in the name of R. Mair: "They sang a song." And R. Zutra b. Tubiah, in the name of Rab, said: "They have straightened their faces to look upon the ark, and sang a song." What was their song? R. Jochanan in the name of R. Mair said (Ex. 15, 1) Then sang Moses, etc. R. Jochanan himself, however, said (Is. 12, 4) And ye shall say on that day, Give thanks unto the Lord, call on His name, etc. And R. Simon b. Lakish said: "The song the orphan chapter (a chapter in which the author's name is not mentioned) (Ps. 98, 1) Oh sing unto the Lord a new song; for He Hath done marvelous things, etc." And R. Elazar said (Ib. 99, 1) The Lord reigneth; let the people tremble. R. Samuel b. Nachmeni said (Ib. 93, 1) The Lord reigneth; He is clothed in majesty. R. Isaac Naphcha said: Sing, O ark, in beauty shining; Thou adorned with chains of gold — ever close the Word enshrining — glittering with gems untold." R. Ashi taught the saying of R. Isaac Naphcha in connection with this (Num. 10, 55) And it came to pass, when the ark set forward that Moses said, etc. And what did Israel say? Whereupon R. Isaac said: "Sing, O ark, etc."

What is meant by As they are being covered? R. Levi said: If they glanced into the ark for as long as the blinking of an eye, they would have died at once. You know this is so from the fact that when the Philistines captured the ark and then sought to return it, they took the kine and hitched them in the usual way with their buttocks toward the ark, but the milch kine realized what was expected of them and turned their heads toward the ark and pushed it on the way, as is said: And the kine took the straight way (I Sam. 6:12).