2 Samuel 1:1

Now it came to pass after the death of Saul, when David was returned from the slaughter of the Amalekites, and David had abode two days in Ziklag;

Cross-reference

1 Samuel 27:6 explains that Achish gave Ziklag to David, establishing why David was living there after Saul's death.

1 Samuel 30:17-26 recounts David's victory over the Amalekites and distribution of spoils — the immediate backstory to his return to Ziklag.

1 Chronicles 12:1 lists the mighty men who joined David at Ziklag, providing additional detail about his supporters there.