Musnad Ahmad 28:593

Qutaybah narrated to us, Ibn Lahi'ah narrated to us, from Yazid ibn Abi Habib, from 'Amr ibn al-Walid, from 'Abdullah ibn 'Amr, who said: I asked the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, do you feel the revelation?" The Messenger of Allah, may Allah bless him and grant him peace, said: "Yes, I hear a ringing sound, then I fall silent at that. There is no time that it is revealed to me except that I think my soul is departing."