Musnad Ahmad 17:1

Waki’ and ‘Abd al-Rahman narrated to us, they said: Sufyan narrated to us, from Mus’ab ibn Muhammad, from Ya’la ibn Abi Yahya, from Fatimah bint Husayn, from her father—‘Abd al-Rahman said: Husayn ibn ‘Ali—who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The beggar has a right, even if he comes upon a horse."