مُوَطَّأُ مَالِكٍ
Muwatta Malik 334
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّهُ سَأَلَ سَالِمَ بْنَ عَبْدِ اللَّهِ هَلْ يُجْمَعُ بَيْنَ الظُّهْرِ وَالْعَصْرِ فِي السَّفَرِ فَقَالَ نَعَمْ لاَ بَأْسَ بِذَلِكَ أَلَمْ تَرَ إِلَى صَلاَةِ النَّاسِ بِعَرَفَةَ
Yahya related to me from Malik from Ibn Shihab that he had asked Salim ibn Abdullah, "Can you join dhuhr and asr when travelling?" He said, "Yes, there is no harm in that. Haven't you seen the people pray on Arafa?"
Arabic/English book reference: Book 9, Hadith 6