Yahya related to me from Malik from Ata al-Khurasani that heheard Said ibn al Musayyab say, "A traveller who has decided to remainsomewhere for four nights does the prayer in full."Maliksaid, "That is what I prefer most out of what I have heard."Malik when asked about the prayer of a prisoner, said, "It is the sameas the prayer of a person who remains in one place, except if he istravelling."
USC-MSA web (English) reference: Book 9, Hadith 20
Arabic reference: Book 9, Hadith 348