What is the primary, textual definition of the travel distance (Safar) that alters resident rulings in the Hanafi school?

The Traveler's Prayer: Distance, Qasr, and Types of Residence

• Exactly eighty-eight (88) kilometers.
• A journey of three days and three nights at a moderate pace.
• Exactly forty-eight (48) English miles.
• Walking until one crosses the nearest river.