Explanation of the hadith
‘Abdullāh ibn Ash-Shikhkhīr (may Allah be pleased with him) said that he saw the Prophet (may Allah's peace and blessings be upon him) praying, and he could hear a sound from him similar to that of a mill. That sound made by the mill is produced by grinding. The Companion,(may Allah be pleased with him) likened the Prophet's weeping sound to the rumbling of the mill. This is how the Prophet (may Allah's peace and blessings be upon him) was with his Lord, though Allah has forgiven his previous and late sins. However, he was the most pious of people, the most conscious of Allah, and the one who feared Allah, the Exalted, the most owing to his perfect knowledge of his Lord.