(1) باب صَلاَةِ الْمُسَافِرِينَ وَقَصْرِهَا
Yahya b. Yazid al-Huna'i reported:
I asked Anas b. Malik about shortening of prayer. He said: When the Messenger of' Allah (ﷺ) had covered a distance of three miles or three farsakh (Shu'ba, one of the narrators, had some doubt about it) he observed two rak'ahs.
Reference: Sahih Muslim 691
In-book reference: Book 6, Hadith 14
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1471