Sayyidah Aishah (may Allah be pleased with her) said: When the Prophet (peace and blessings be upon him) prayed, his blessed feet would swell. Sayyidah Aishah (may Allah be pleased with her) said: O Prophet of Allah! Why do you do this, when Allah has forgiven your past and future sins? He (peace and blessings be upon him) said: "O Aishah! Should I not be a grateful servant of Allah?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13274