Sayyid Ibn Tawus [may his soul be sanctified] quotes from 'Ali ibn al-Husayn (AS):
He [may his soul be sanctified], quoting az-Zahri, in Fathul Abwab, says:
"I, together with 'Ali ibn al-Husayn (AS), called upon 'Abdul Malik ibn Marwan. 'Abdul Malik, regarding it as magnificent to see the mark of prostration between the two eyes of 'Ali ibn al-Husayn (AS), said: `O Abu Muhammad, perseverance is obvious on your face, and Allah has already been beneficent to you, as you are a part of the Messenger of Allah (SA), a close relative with a firm relation to him, and you have a great favour upon your family and your contemporaries. You have been given such merits, knowledge, godliness and piety that have been given to no one like you or before you, except your past ancestors.' He continued praising and extolling him.
'Ali ibn al-Husayn (AS) said: `All that which you have described is of Allah's favour, support and grace. But how is [my] thanking Him on what He has favoured [me], O Amirul Mu'minin? The Messenger of Allah (SA) used to stand in his salat on his feet till they got swollen, and he remained thirsty during his fasting such that his mouth would dry up. He was asked: `O Messenger of Allah! `Did not Allah forgive what has passed and what is to come of your sins?' He replied: `Am I not to be a thankful servant?' Praise be to Allah for what He has favoured us with and for His testing us, and praise be to Him in the Hereafter and in this world. By Allah, even if my limbs were cut to pieces and my eyeballs ran down my chest just to pay tribute to Allah, the Most Glorified, for just a tenth of one tenth of a single one of all His favours, which no reckoner can reckon, nor the praise of the praisers can pay the tribute due to a single one of them, I could not pay the due tribute to Him. Never, by Allah, unless He grants me that nothing should keep me from praising and remembering Him all days and nights, secretly and publicly. Had I not been under obligations to my family and to other people, in particular and in general, who have rights against me, and which I cannot but fulfil to them as it is possible and as is in my capacity, I would have turned my eye to the heaven, and my heart to Allah, and then I would have never taken them back till He would take my breath-He is the best of judges.' Then he (AS) wept, and so did 'Abdul Malik ... " as the hadith goes.
Biharul Anwar, vol. 46, p. 57, "Fathul Abwab"