Musnad Ahmad 626:105

Waki’ narrated to us, from Mis’ar and Sufyan, from Ziyad ibn ‘Alaqah, from Al-Mughirah ibn Shu’bah: That the Prophet, peace and blessings of Allah be upon him, used to pray until his feet would swell. It was said to him [regarding that], and he said: "Shall I not be a grateful servant?"