Musnad Ahmad 30:2615

Waki’ narrated to us, Sa’dan al-Juhani narrated to us, from Abu Mujahid, from Abu Mudillah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Three whose supplication is not rejected: the just leader, the fasting person until he breaks his fast, and the supplication of the oppressed. Allah raises it above the clouds on the Day of Resurrection, and the gates of the heaven are opened for it, and the Lord (Mighty and Majestic is He) says: 'By My Might, I will surely aid you, even if it be after a while.'"