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: 5767
'Ubida b. Samit reported that when wahi descended upon Allah's Apostle (may peace be upon him), he lowered his head and so lowered his Companions their heads, and when (this state) was over, he raised his head.