Muwatta Malik Book 9, Hadith 21
Yahya related to me from Malik from Ata al-Khurasani that he
heard Said ibn al Musayyab say, "A traveller who has decided to remain
somewhere for four nights does the prayer in full."
Malik
said, "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 same
as the prayer of a person who remains in one place, except if he is
travelling."
Yahya related to me from Malik from Nafi that Ibn Umar stayed in Makka for ten nights, shortening the prayer, except when he prayed it behind an imam, in which case he followed the imam's prayer.
Yahya related to me from Malik from Ibn Shihab from Salim ibn Abdullah from his father that Umar ibn al-Khattab, when he went to Makka, used to lead them in prayer and do two rakas and then say, "People of Makka,complete the prayer, we are a group travelling."