Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "There are three people whose supplication it is binding upon Allah Almighty not to reject: the fasting person until he breaks his fast, the oppressed person until he obtains help (that is, redress), and the traveller until he returns."
Imam al-Tirmidhi recorded this narration with the part about the traveller abridged, and this wording is not in it.
Imam al-Bazzar recorded this narration.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدعية / 17228