Within the Arjunabhigamana पर्व of the Aranya पर्व of महाभारत, one of the famous श्लोक on क्षमा is captured below. This is conveyed by Yudhistira to Draupadi when she questions on how he does not experience anger and why he is considering to give the Kauravas who have behaved in a wicked manner against the Pandavas. These श्लोक are famous – hence reproduced below. 

क्षमाधर्मः क्षमा यज्ञ: क्षमा वेदा: क्षमा श्रुतम्। य एतदेवं जानाति स‌ सर्वं क्षन्तुमहति।।

क्षमा is धर्म, क्षमा is यज्ञ, क्षमा is वेद, क्षमा is श्रुति, he who knows all this is capable of forgiving all.

क्षमा ब्रह्म क्षमा सत्यं क्षमा भूतं च भावि च। क्षमा‌ तप: क्षमा शौचं क्षमयेदंधृतं जगत्।।

क्षमा is ब्रह्म क्षमा is सत्य, क्षमा is the accumulated and future merit, क्षमा‌ is तप,  क्षमा is purity and by क्षमा‌, universe is sustained. 

अति यज्ञविदां‌ लोकान् क्षमिण: प्राप्नुवन्ति च। अति ब्रह्मविदां लोकान् अति चापि‌ तपस्विनाम्।।

Those having nature of क्षमा will attain the लोक of those who are experts in यज्ञ क्रिया, those who are experts is ब्रह्मविद्या and those who are expert तपस्वी

अन्ये वै यजुषां लोका: कर्मिणामपरे तथा। क्षमावतां ब्रह्मलोके लोका: परमपूजिता:।।

Those who are experts in conduct of वैदिक कर्म and other कर्म will attain other regions while those who practice क्षमा‌ attain to those परम पूजित ब्रह्मलोक 

क्षमा तेजस्विनां तेज: क्षमा ब्रह्म तपस्विनाम्। क्षमा सत्यं सत्यवतां क्षमा यज्ञ: क्षमा शम:।।

क्षमा is the तेज within the persons who are तेजस्वी, क्षमा is the ब्रह्म present within तपस्वी, क्षमा‌ is the सत्य (truth) within those who are सत्यवान्, क्षमा is यज्ञ, क्षमा represents control of the mind

तां क्षमां तादृशीं कृष्णे कथमस्मद्विधस्त्यजेत्। यस्यां ब्रह्म च सत्यं च यज्ञा लोकाश्च धिष्ठिता:।।

How can a person like me, O कृष्णा, give up such क्षमा in which are established ब्रह्म, सत्य, यज्ञ and the various (higher) लोक

क्षन्तव्यमेव सततं पुरुषेण विजानता। यदा हि क्षमते सर्वं ब्रह्म सम्पद्यते तदा।।

Even knowing all, a person must pursue क्षमा, whosoever forgives everything attains to ब्रह्म.

क्षमावतामयं लोक: परश्चैव क्षमावताम्। इह सम्मानमृच्छन्ति परत्र च शुभाम् गतिम्।।

This world belongs to one who practices क्षमा, the other world (after death) also belongs to one who practices क्षमा, thet obtain honour in this world as well as शुभम् in the other world.

येषां मन्युर्मनुष्याणां क्षमयाभिहत: सदा। तेषां परतरे लोकास्तस्मात् क्षान्ति: परा मया।।

Those persons, whose anger is overpowered by क्षमा attain to the higher लोक, therefore क्षमा is considered the highest धर्म

इति गीता: काश्यपेन गाथा नित्यं क्षमावताम्। श्रुत्वा गाथा: क्षमायास्त्वम् तुष्य द्रौपदि मा  क्रुद्धः:।।

These verses were always chanted by ऋषि काश्यप in the honour of people practicing क्षमा, be pleased and be not angry, O द्रौपदि. 

Source

Translation of Mahabharata by MN Dutt