Musnad Abu Ya'la al-Mawsili 165:13

Abu Khaythama narrated to us, Waki' narrated to us, Sufyan narrated to us, from Mus'ab bin Muhammad, from Ya'la bin Abi Yahya, from Fatimah bint Husayn, from her father, 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."