Muwatta Imam Malik Book of Shortening the Prayer, Hadith 20

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."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ عَطَاءٍ الْخُرَاسَانِيِّ، أَنَّهُ سَمِعَ سَعِيدَ بْنَ الْمُسَيَّبِ، قَالَ مَنْ أَجْمَعَ إِقَامَةً أَرْبَعَ لَيَالٍ وَهُوَ مُسَافِرٌ أَتَمَّ الصَّلاَةَ . قَالَ مَالِكٌ وَذَلِكَ أَحَبُّ مَا سَمِعْتُ إِلَىَّ . وَسُئِلَ مَالِكٌ عَنْ صَلاَةِ الأَسِيرِ فَقَالَ مِثْلُ صَلاَةِ الْمُقِيمِ إِلاَّ أَنْ يَكُونَ مُسَافِرًا .