Musannaf Ibn Abi Shayba 4:150

Umar ibn Ayyub narrated to us, from Ja'far ibn Burqan, who said: I asked al-Zuhri, "In what [distance] do you shorten the prayer?" He said, "In the distant journey (al-safar al-mu'in)." I said, "And what is the 'distant' (al-im'an) in your opinion?" He said, "Two days."