(2) باب مَتَى يُقْصِرُ الْمُسَافِرُ
Narrated Yahya b. Yazid al-Hannani:
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.
Reference: Sunan Abi Dawud 1201
In-book reference: Book 4, Hadith 4
English translation: Book 4, Hadith 1197