While staying in the miracle cave Jetsun Milarepa said to Rechungpa (His main disciple):

“You’ve obtained the precepts of the Dakinis’ Ear-Whispered Tantras to
complete the transmission of my instructional lineage.
Now you must practice them to achieve results in this lifetime.”

Rechungpa asked him:
“Please sing me a song expressing the key for obtaining the supreme siddhi (enlightenment) in this lifetime.”

Jetsun replied, “My ultimate precept is this,” and he turned around, exposing his buttocks,
which were prominently covered with lumps of hard callus from long periods of sitting meditation.

Seeing this, Rechungpa was overwhelmed with immense admiration and respect for the austerities in practice endured by his lama.
Tears welled up in his eyes and he thought with conviction, “I, too, must practice like this.”