From: Rabbi Moshe Sofer
To: Rabbi Shalom
The Chasam Sofer finds the scriptural basis for the Jewish practice of avoiding even truthful oaths in Koheles 9:2, which contrasts the habitual swearer with the praiseworthy person who fears oaths entirely. He concludes that this custom of oath-avoidance carries the status of Torah (minhag Yisrael Torah) and harmonizes the apparently conflicting explanations of Rashi and the Sma by showing they address different scenarios — false versus truthful oaths.
Source: Shu"t Chasam Sofer CM §90