I asked Anas b. Malik about the shortening of the prayer (while travelling). He said: When the Messenger of Allah ﷺ went out on a journey of three miles or three farsakh (the narrator Shu'bah doubted), he used to pray two rak'ahs.  

أَنَسٌ كَانَ رَسُولُ اللَّهِ ﷺ إِذَا خَرَجَ مَسِيرَةَ ثَلاَثَةِ أَمْيَالٍ أَوْ ثَلاَثَةِ فَرَاسِخَ شُعْبَةُ شَكَّ يُصَلِّي رَكْعَتَيْنِ  

I prayed along with the Messenger of Allah ﷺ four rak'ahs at the noon prayer at Medina and two rak'ahs at the afternoon prayer in Dhu al-Hulaifah.  

عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ وَإِبْرَاهِيمَ بْنِ مَيْسَرَةَ سَمِعَا أَنَسَ بْنَ مَالِكٍ يَقُولُ صَلَّيْتُ مَعَ رَسُولِ اللَّهِ ﷺ الظُّهْرَ بِالْمَدِينَةِ أَرْبَعًا وَالْعَصْرَ بِذِي الْحُلَيْفَةِ رَكْعَتَيْنِ