Musnad Ahmad 30:3066

Waki’ narrated to us, he said: Hisham al-Dastawa’i narrated to us, from Yahya ibn Abi Kathir, from Abu Ja’far, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Three supplications are answered, there is no doubt in them: the supplication of the oppressed, the supplication of the parent, and the supplication of the traveler."