宋高僧傳 13.32

13.32

系曰：凡夫捨報，尸必一同也。佛則右脇，果位坐亡，首搘地者現通，身立中者彰異。其惟欲行步而化者，除後僧會外，則因公有焉。

The appraisal states: When ordinary mortals relinquish their karmic allotment, their corpses are inevitably identical. But the Buddha lay on his right side, those who have attained the fruits of the path die sitting, those with heads propping on the ground manifest supernormal powers, and those whose bodies stand erect display the extraordinary. As for dying while walking, except for the later Senghui, only Master Yin possessed this.