Sahih Muslim
The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fadail)
Book Number: 030 / Hadith Number: 5766
'Ubida b. Samit reported that when wahi (inspiration) descended upon Allah's Messenger (may peace be upon him), he felt a burden on that account and the colour of his face underwent a change.