I heard this shloka quoted repeatedly by Swami Premanandaji Maharaj of Brindavan in his talks. I searched the source and found it here in Barhma Vaivarta Purana
नाभुक्तं क्षीयते कर्म कल्पकोटिशतैरपि ।
अवश्यमेव भोक्तव्यं कृतं कर्म शुभाशुभम् ॥ १७ ॥
Meaning:
Until exhausted, Karma does not diminish or disappear, even after hundreds of crores of epochs. It has to be experienced through its fruit as an amelioration, without fail, with good and bad fruits accruing to good and bad actions.
