73:2
“Stand all night [in prayer] except a little”
Beginning of Revelation in the Cave of Hira
Al-Muzzammil is among the earliest-revealed surahs, instituting qiyam al-layl in the first days after the beginning of revelation.
73:5
“We will soon send upon you a weighty revelation.”
Beginning of Revelation in the Cave of Hira
Cited as one possible explanation for why Jibril squeezed the Prophet ﷺ during the first revelation -- to indicate the weighty revelation about to come.