Allah's Apostle said, "May Allah's mercy be on him who is lenient in his buying, selling, and in demanding back his money."
Sahih Bukhari: Volume 3, Book 34, Number 290
Some people asked Allah's Apostle, "Whose Islam is the best? i.e. (Who is a very good Muslim)?" He replied, "One who avoids harming the Muslims with his tongue and hands."
Volume 1, Book 2, Number 10