Adam Clarke on Isaiah 37:21

Then Isaiah - sent unto Hezekiah - The Syriac and Septuagint understand and render the verb passively, was sent.

Whereas thou hast prayed to me against Sennacherib "Thy prayer unto me concerning Sennacherib - I have heard" - שמעתי shamati; this word, necessary to the sense, is lost in this place out of the Hebrew text. One MS. of Dr. Kennicott's and one of De Rossi's have it written above the line in a later hand. The Septuagint and Syriac found it in their copies; and it is preserved in the other copy; 2Kgs 19:20.