Chapter on Sales
Man Lā Yaḥḍuruh al-Faqīh|Volume 3|Book 2|Chapter 11
وَ رُوِيَ عَنْ عَبْدِ اَلرَّحْمَنِ بْنِ اَلْحَجَّاجِ قَالَ: سَأَلْتُ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ عَنْ فُضُولِ اَلْكَيْلِ وَ اَلْمَوَازِينِ فَقَالَ "إِذَا لَمْ يَكُنْ تَعَدَّى فَلاَ بَأْسَ ".
It is narrated from Abd Al-Rahman ibn Al-Hajjaj, who said: I asked Abu Abdillah عليه السلام about the excess that may result from measuring or weighing. Imam عليه السلام said: "If there is no deliberate excess (beyond fairness), then there is no harm."
- Ṣaḥīḥ al-Faqīh, p. 221