The Messenger of Allah (ﷺ) gave him that wood as a protected land.
When Umar ibn al-Khattab succeeded, Sufyan ibn Wahb wrote to Umar asking him about this wood. Umar ibn al-Khattab wrote to him: If he (Hilal) pays you the tithe on honey what he used to pay to the Messenger of Allah (ﷺ), leave the protected land of Salabah in his possession; otherwise those bees are like those of any wood; anyone can take the honey as he likes.
In-book : Book 9, Hadith 45
English translation : Book 9, Hadith 1596