'Abdullah bin Shikh-kheer Radiyallahu 'Anhu says:
"I attended the noble assembly of Rasulullah Sallallahu 'Alayhi Wasallam. He was performing salaah. Because of his crying, such sound emmitted from his chest, like that of a boiling pot".
English reference: Book 44, Hadith 305
Arabic reference: Book 45, Hadith 322