Al-Kāfi - Volume 5 > Borrowing Dirhams (Coins) to Keep the Good ones Among them
Hadith #3

3- عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ حَمَّادٍ عَنِ الْحَلَبِيِّ عَنْ أَبِي عَبْدِ اللهِ (عَلَيْهِ السَّلام) قَالَ إِذَا أَقْرَضْتَ الدَّرَاهِمَ ثُمَّ أَتَاكَ بِخَيْرٍ مِنْهَا فَلا بَأْسَ إِذَا لَمْ يَكُنْ بَيْنَكُمَا شَرْطٌ.

3. Ali ibn Ibrahim has narrated from his father from ibn abu ‘Umayr from Hammad from al-Halabiy who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘If you give dirhams as a loan, then the party pays you back with better ones, it is not unlawful, if there is no stipulation between you.’”