«كان إذا أنزل عليه الوحي نكس رأسه ونكس أصحابه رءوسهم فإذا أقلع عنه رفع رأسه»
It was the practice of the Prophet (peace and blessings be upon him):
"When revelation was sent down upon him (peace and blessings be upon him), he (peace and blessings be upon him) would lower his head, and his (peace and blessings be upon him) Companions would also lower their heads; then, when the revelation ceased, he (peace and blessings be upon him) would raise his head."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4686