Musnad Ahmad 28:254

'Abd al-Samad narrated to us, from 'Abd Allah ibn al-Mubarak, [who said] Usamah ibn Zayd narrated to us, from 'Amr ibn Shu'ayb, from his father, from 'Abd Allah ibn 'Amr, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The zakat (charity) of the Muslims shall be collected at their watering places."