Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: When Jibril (peace be upon him) brought the revelation, (the Prophet (peace and blessings be upon him)) would not be finished until he (peace and blessings be upon him) was wrapped up because of the revelation, and the Prophet (peace and blessings be upon him) would keep repeating (hurriedly) the beginning of the revelation, out of fear that he (peace and blessings be upon him) would lose consciousness. Jibril (peace be upon him) asked him (peace and blessings be upon him): "Why do you do this?" He (peace and blessings be upon him) said: "Out of fear that I will forget." So Allah, the Mighty and Majestic, revealed this verse: "We will make you recite, and you will not forget" [Al-A'la: 6].
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12595