Silsilat al-Ahadith al-Sahihah

Yahya bin Yazid Hanai says that I asked Sayyiduna Anas bin Malik (may Allah be pleased with him) about the shortened prayer (qasr), because when I traveled towards Kufa, I would perform two rak‘ahs (for Zuhr, Asr, and Isha) until I returned. He said: When the Messenger of Allah (peace and blessings be upon him) would travel a distance of three miles or three farsakhs, he would perform the shortened prayer. And in one narration, it is mentioned that he would perform two rak‘ahs. The narrator of the hadith, Imam Shu‘bah, was uncertain whether it was miles or farsakhs.