Musnad Ahmad 28:307

Isma'il ibn Muhammad ibn Jahadah narrated to us, Hajjaj narrated to us, from 'Amr ibn Shu'ayb, from his father, from his grandfather, who said: I saw the Prophet, peace and blessings of Allah be upon him, turning from his right and from his left in prayer, and drinking while standing and while sitting, and praying barefoot and while wearing sandals, and fasting during travel and breaking the fast.