Hadithcore
Mishkat al-Masabih, 1351

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 <i>al-Muwatta’</i>.

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

References1 variant
In-Book Reference
Book 4, Hadith 758