Albert Barnes Commentary 1 Kings 1:21

"Otherwise it will come to pass, when my lord the king shall sleep with his fathers, that I and my son Solomon shall be counted offenders." — 1 Kings 1:21 (ASV)

Shall sleep - This euphemism for death, rare in the early Scriptures—being found only once in the Pentateuch (as noted in a margin reference) and once also in the historical books before Kings (2 Samuel 7:12)—becomes in Kings and Chronicles the ordinary mode of speech (see 1 Kings 2:10; 1 Kings 11:43; 2 Chronicles 9:31; 2 Chronicles 12:16; etc.).

David uses the metaphor in one psalm (Psalms 13:3). In the later Scriptures it is, of course, common (Daniel 12:2; Matthew 9:24; John 11:11; 1 Corinthians 11:30; 1 Corinthians 15:51; 1 Thessalonians 4:14; etc.).