Solomon's public work was largely carried out by the jinns. This was a punishment for their sins of making people believe that they were all powerful, knew the unseen, and could foresee the future. As a prophet, it was Solomon's duty to remove such false beliefs from his followers.
Related Quran Ayaaths of this Hadith.
Surah 6 : Ayah 84 Surah 4 : Ayah 163 Surah 27 : Ayah 15 Surah 27 : Ayah 16 Surah 27 : Ayah 17 Surah 27 : Ayah 18 Surah 27 : Ayah 19"When a person dies, his works end, except for three: ongoing charity, knowledge that is benefited from, and a righteous child who prays for him."
Prophet Mohammed (PBUH)
"The best of what a man leaves behind are three: a righteous child who supplicates for him, ongoing charity the reward of which reaches him, and knowledge that is acted upon after him."
Sunan Ibn Mājah
"Every day two angels come down from Heaven and one of them says, 'O Allah! Compensate every person who spends in Your Cause,' and the other (angel) says, 'O Allah! Destroy every miser.'"
Sahih Bukhari