Rabbi Jacob bar Zavday, in the name of Rabbi Abbahu, opened his discourse with the verse: "And it shall no longer be for the house of Israel a source of confidence that brings iniquity to remembrance" (Ezekiel 29:16). And it is written: "Seraphim were standing above Him, and so forth" (Isaiah 6:2). "With two he covered his face" — so as not to gaze; "and with two he covered his feet" — so that they would not see the face of the Shekhinah, for it is written: "And the sole of their feet was like the sole of a calf's foot" (Ezekiel 1:7). This is on account of "And it shall no longer be for the house of Israel a source of confidence that brings iniquity to remembrance."
Why the Seraphim Hide Their Calf-Shaped Feet from the Shekhinah
Midrash Tanchuma Buber, Emor 11:1
ר' יעקב בר זבדי בשם ר' אבהו פתח ולא יהיה עוד לבית ישראל למבטח מזכיר עון (יחזקאל כט טז), וכתיב שרפים עומדים ממעל לו וגו' (ישעיה ו ב), בשתים יכסה פניו שלא להביט בשינה, ובשתים יכסה רגליו שלא יראו פני השכינה, דכתיב וכף רגליהם ככף רגל עגל (יחזקאל א ז), על שם ולא יהיה עוד לבית ישראל למבטח מזכיר עון.