Tahdhīb al-Aḥkām 2, record 67 of the digital edition

Translation

That which was narrated by Muhammad ibn Ya’qub, from Ali ibn Ibrahim, from his father, from Salih ibn Sa’id, from Yunus, from some of his men, from Abu Abdillah (a.s), who said: I asked Him (a.s), peace be upon him, about what came in the hadith: ‘Pray ‘Asr when the sun is at one qamah (height), two qamahs, one dhira (forearm), two dhiras, one qadam (foot), two qadams, from this and from that’, so when is this, and how is this, while at some times the shadow is only half a qadam?” Imam (a.s) said: ‘Indeed, he said ‘the shadow of the qamah’ and did not say ‘the qamah of the shadow’, because the shadow of the qamah varies, sometimes it increases, sometimes it decreases, while the qamah (the standing height) remains a qamah forever, it does not change.’ Then Imam (a.s) said: ‘One dhira and two dhiras, one qadam and two qadams, so the one dhira and two dhiras are the explanation of the qamah and the two qamahs during the time when the shadow of the qamah is one dhira, and the shadow of the two qamahs is two dhiras. Thus, the shadow of the qamah and the two qamahs, and the dhira and the two dhiras, are consistent in every time, known, and explained, one interpreted by the other, supported by it. So when the time is such that the shadow of the qamah is a dhira, then the time is a dhira of the shadow of the qamah, and the qamah is a dhira of the shadow. And if the shadow of the qamah is less or more, then the time is confined to the dhira and the two dhiras. This is the explanation of the qamah and the two qamahs, and the dhira and the two dhiras.”’

Translation: Haydar Ali Shaykh ibn Muhammad — published with the edition, reproduced verbatim.