It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that Surah Mursalat was revealed to the Prophet (peace and blessings be upon him) on the night of the snake. We asked him, "What is meant by the night of the snake?" He replied, "Once, at night, we were with the Prophet (peace and blessings be upon him) in the cave of Hira when suddenly a snake came out from the mountain. The Prophet (peace and blessings be upon him) ordered us to kill it. We quickly advanced, but it outpaced us and entered its hole. The Prophet (peace and blessings be upon him) said: 'Leave it, it has escaped your harm just as you have escaped its harm.'"