Musnad Ahmad 30:2478

Yahya narrated to us, from Hisham, Yahya narrated to us, from Abu Ja'far, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Three supplications are not to be doubted: the supplication of the traveler, the supplication of the oppressed, and the supplication of the parent against their child."