Musannaf Ibn Abi Shayba 3:5970

Waki’ narrated to us, from al-A’mash, from Abu Salih, from a man among the companions of the Prophet, may Allah bless him and grant him peace: The Prophet, may Allah bless him and grant him peace, used to stand in prayer until his feet would swell. It was said to him [regarding that], and he said, "Should I not be a grateful servant?"