بُلُوغُ الْمَرَامِ
Bulugh al-Maram 912
Business Transactions
وَعَنْ اِبْنِ عَبَّاسٍ - رضى الله عنه - قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ إِنَّ أَحَقَّ مَا أَخَذْتُمْ عَلَيْهِ حَقًّا كِتَابُ اَللَّهِ } أَخْرَجَهُ اَلْبُخَارِيُّ [1] .
Narrated Ibn 'Abbas (RA): Allah's Messenger (ﷺ) said: "The most worthy thing for which you receive payment is Allah's Book." [Reported by al-Bukhari].
Sunnah.com reference: Book 7, Hadith 164