What serves as a reminder for the fire of Hell is the fire of this world. Allah Almighty says:
“We have made it [fire] a reminder and provision for the travelers.”[Al-Waqi`ah 56:73]
Meaning that Allah has made the fire of this world a reminder that reminds one of the Fire of the hereafter.
[Fath al-Bari, Ibn Rajab (2/343)]