www.lesswrong.com/posts/aGnRcBk4rYuZqENug/it-s-okay-to-feel-bad-for-a-bit

The attribution to the Dhammapada is not correct.

• A complete public-domain text of the Dhammapada is a short anthology of 423 verses in 26 chapters. In its chapter on the arhat ("Chapter VII. The Venerable (Arhat)"), the relevant verses are things like: "There is no suffering for him who has finished his journey, and abandoned grief" and related short verses. The quoted sentence about an arahant being struck, feeling only physical pain, and organizing his mother's funeral does not appear in the Dhammapada text.
• The recognizable clause "He feels one pain: physical, but not mental" is found instead in the Sallatha Sutta (SN 36.6), which says of the well-instructed disciple: "So he feels one pain: physical, but not mental."
• The added example "If his mother died, he would organize the funeral, but would feel no grief, no sense of loss" is not in the Dhammapada text fetched below, and it is not part of the Sallatha Sutta passage either. So this is not a Dhammapada quotation.

In short: the post presents this as a quotation from the Dhammapada, but the source text does not contain it; the most identifiable part comes from a different Buddhist scripture entirely.