Musnad Ahmad 25:1106

Yahya ibn Adam narrated to us, Abu Bakr ibn Ayyash narrated to us, from Idris ibn Munabbih, from his father Wahb ibn Munabbih, from Ibn Abbas, who said: "The Prophet, may Allah bless him and grant him peace, asked Gabriel to let him see him in his [true] form. He said: 'Call upon your Lord.' He said: 'So he called upon his Lord.' He said: 'Then a blackness appeared to him from the direction of the east.' He said: 'It began to rise and spread.' He said: 'When the Prophet, may Allah bless him and grant him peace, saw it, he fell unconscious. So he [Gabriel] came to him, lifted him up, and wiped the saliva from his cheeks.'"