Sunan at-Tirmithiy كتاب الصلاة باب ما جاء فى وضع اليدين على الركبتين فى الركوع
The Messenger of God, peace and blessings of God be upon him, prayed until his feet became swollen. So he was told, "Do you endure this, though you have been forgiven all of your past and future sins?" He replied, "Should I not be a grateful bondsman?" Click on ḥadīth to read more