A certain person asked A'ishah ( Radhiyallaho anha) :

"Tell me something noteworthy concerning the Prophet (Sallallaho alaihe wasallam)."

She  answered :

"There was nothing which was not unusual about him. Everything he did was noteworthy. One night he came and lay down with me. After sometime, he got up saying, 'Now let me pray to my Lord, the Sustainer."

With this, he stood up in Salaat, humbling himself before his Creator with such sincerity that tears rolled down his cheeks to his beard and on to his breast. He then bowed for Ruku' and Sajdah, and his tears flowed down as fast as before and after raising his head from his Sajdah, he continued weeping in this manner till Hadhrat Bilal (Radiyallaho Anha) announced the approach of Fajr. I pleaded with him :

"O, Prophet of Allah ! you are sinless, as Allah has in His munificence forgiven your each and every sin (even if committed) in the past and which may happen in the life to come (XL VIII : 2) and still you grieve so much." He replied : Why, then, should I not be a grateful slave of Allah ? Then he added, "Why should I not be praying like this when Allah has today revealed to me these verses ?"

"Lo ! in the creation of the Heavens and the Earth, and in the differences of night and day, are tokens (of His Sovereignty) for men of understanding, such as remember Allah, standing, sitting and reclining.....                                    (III : 190-191)"

It has been reported in many Ahadith that the Prophet's feet would get swollen because of his very long rakaats in Salaat; people tried to reason with him :

"O Prophet of Allah ! You are sinless and still you labour so hard !"

He would reply : "Should I not be a grateful slave of my Allah, then ?"