Musnad Ahmad 25:1392

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, he said: And 'Uthman al-Jazari informed me that Miqsam, the freed slave of Ibn 'Abbas, informed him, from Ibn 'Abbas, regarding His saying: "And when those who disbelieved plotted against you to restrain you" [Al-Anfal: 30]. He said: "Quraysh consulted one night in Makkah. Some of them said: 'When morning comes, restrain him with bonds,' meaning the Prophet, may Allah bless him and grant him peace. Some of them said: 'Rather, kill him.' And some of them said: 'Rather, expel him.' Allah, the Almighty and Majestic, informed His Prophet of that. So 'Ali spent the night on the bed of the Prophet, may Allah bless him and grant him peace, that night, and the Prophet, may Allah bless him and grant him peace, went out until he reached the cave. The polytheists spent the night guarding 'Ali, thinking he was the Prophet, may Allah bless him and grant him peace. When they reached the morning, they rushed toward him. When they saw 'Ali, Allah turned back their plot. They said: 'Where is this companion of yours?' He said: 'I do not know.' So they followed his tracks. When they reached the mountain, the trail became confused for them. They climbed the mountain and passed by the cave, and they saw a spider's web on its entrance. They said: 'If he had entered here, there would not be a spider's web on its entrance.' So he remained in it for three nights."