Mutarrif narrates from his father, Abdullah bin al-Shikhkhir (may Allah be pleased with him): I saw the Messenger of Allah (peace and blessings be upon him) praying, and because of his (peace and blessings be upon him) weeping, a sound like that of a turning millstone was coming from his chest.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3356