Al-Kāfi - Volume 3 > Where to Apply Rules about Salah (Prayer) on a Journey
Hadith #4

4ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنْ حَمَّادٍ عَنْ حَرِيزٍ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ قَالَ سَأَلْتُ أَبَا عَبْدِ الله (عَلَيْهِ الْسَّلام) عَنْ رَجُلٍ يَدْخُلُ مِنْ سَفَرِهِ وَقَدْ دَخَلَ وَقْتُ الصَّلاَةِ قَالَ يُصَلِّي رَكْعَتَيْنِ فَإِذَا خَرَجَ إِلَى سَفَرٍ وَقَدْ دَخَلَ وَقْتُ الصَّلاَةِ فَلْيُصَلِّ أَرْبَعاً.

4. Ali ibn Ibrahim has narrated from his father from Hammad, from Hariz from Muhammad ibn Muslim who has said the following: “I once asked abu ‘Abd Allah, ‘Alayhi al-Salam, about a man who returns from his journey when it has become the time for a Salah (prayer). He (the Imam) said, ‘He is required to perform his Salah (prayer) as two Rak‘at and if he goes out on a journey when it has become the time for Salah (prayer) he is required to perform his Salah (prayer) as four Rak‘at.’”