2 Samuel 3:37
For all the people and all Israel understood that day that it was not of the king to slay Abner the son of Ner.
Cross-reference
2 Samuel 16:7
Contrast
2 Samuel 16:7 has Shimei cursing David as a 'man of blood' — directly contradicting the people's understanding of David's innocence here.
1 Kings 2:32
Historical context
1 Kings 2:5 recounts Joab's murder of Abner — confirming what 3:37 implies: David had no part in Abner's death.