Musar על שבת 19:2

However, this fear must first grow strong in one's heart and afterwards its effects will also manifest in the limbs of the body, namely, a bowed head and prostration, lowering of the eyes and clasping of the hands, as a lowly slave before a great king. Likewise, they said in the Gemara "Rava clasped his hands and prayed, saying, 'I am like a slave before his Master'" (Shabbat 10a).

They further said (Shabbat 10a): "Rava would don a cloak and pray, saying: 'prepare to meet your G-d, O Israel' (Amos 4:12)".