Musnad Ahmad 30:3640

'Abd al-Malik ibn 'Amr narrated to us, Hisham narrated to us, from Yahya, from Abu Ja'far, who said: I heard Abu Hurayrah saying: 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 traveler, and the supplication of the parent against their child."