ENOfficial
Malik said he heard that Ibn ‘Abbas used to shorten the prayer on a journey equivalent to that between Mecca and at-Ta'if, or between Mecca and ‘Usfan, or between Mecca and Judda. Malik said that that was four postal stages. He transmitted it in al-Muwatta’.
Source: sunnah.com
Other translations
Arabic
وَعَن مَالك بَلَغَهُ أَنَّ ابْنَ عَبَّاسٍ كَانَ يَقْصُرُ فِي الصَّلَاة فِي مثل مَا يكون بَين مَكَّة والطائف وَفِي مثل مَا يكون بَيْنَ مَكَّةَ وَعُسْفَانَ وَفَى مِثْلِ مَا بَيْنَ مَكَّةَ وَجُدَّةَ قَالَ مَالِكٌ: وَذَلِكَ أَرْبَعَةُ بُرُدٍ. رَوَاهُ فِي الْمُوَطَّأ
sunnah.com