1 Samuel 30:9
So David went, he and the six hundred men that were with him, and came to the brook Besor, where those that were left behind stayed. So David went, he and the six hundred men that were with him, and came to the brook Besor, where those that were left behind stayed.
David set out with all six hundred men who were with him in pursuit of the Amalekites who had raided Ziklag, and came to the brook Besor. He went in urgent pursuit after receiving the Lord's answer that he should go and would succeed. Hunger, thirst, and lack of rest did not stop the advance, because time was short and every effort was needed.
The verse itself does not say that any stayed behind at this point. Two hundred men became too exhausted, faint, and weary to continue and remained at the brook Besor. Those who stayed behind guarded the supplies and baggage, while David continued the pursuit with the other four hundred.
The march to Besor began the campaign against the Amalekites that ended in victory. That victory was soon followed by David becoming king of Judah.