Al-Kāfi - Volume 3 > Where to Apply Rules about Salah (Prayer) on a Journey
Hadith #8

8ـ عَلِيٌّ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ عَلِيِّ بْنِ يَقْطِينٍ عَنْ أَبِي الْحَسَنِ (عَلَيْهِ الْسَّلام) قَالَ سَأَلْتُهُ عَنْ رَجُلٍ خَرَجَ فِي سَفَرٍ ثُمَّ تَبْدُو لَهُ الإِقَامَةُ وَهُوَ فِي صَلاَتِهِ قَالَ يُتِمُّ إِذَا بَدَتْ لَهُ الإِقَامَةُ.

8. Ali ibn Ibrahim has narrated from his father from ibn abu ‘ Umayr from Ali ibn Yaqtin who has said the following: “I once asked abu al-Hassan, ‘Alayhi al-Salam, about a man who is on a journey and during performing a Salah (prayer) he decides to stay there for ten days. He (the Imam) said, ‘If he decides to stay for ten days, he is required to perform his Salah (prayer) complete.’” (The following chapter is best explained in the form of fatwa.)