Hazrath Anas performed Qasr in a journey of 15 miles. Hazrath Ali is said to have travelled once to a nearby oasis is from where he came back to Madinah on the same day. There, he led people in Qasr Jama at. It is assumed that the place would not have been more than 25 miles from Madinah. According to Sahih Bukhari, Hazrath Ibne Abbas and Ibne Umar performed Qasr and availed leave of fasting in journey of 48 miles.
Scholars also differ in fixation of the distance.
In another narration the distance mentioned is a three days journey . Both these narrations have been cited by Imam Bukhari. In the absence of any direct instruction of a travel-limit for Qasr, the scholar relied on the above Hadith for the definition of a journey. But that is not quite relevant if you think a little. The definition of a journey for both the purposes may not be the same.