From him, from Al-Hasan ibn ‘Udays from Ishaq ibn ‘Ammar from Isma‘il al-Ju‘fi: Abu Ja‘far عليه السلام, peace be upon him, said: "The Messenger of Allah سبحانه وتعالى, may Allah سبحانه وتعالى bless him and his family, when the shadow on the wall measured one arm’s span (dhira‘), would pray Dhuhr, and when it reached two arm’s spans, He صلى الله عليه وعلى آله وسلم would pray ‘Asr." I asked: "The walls differ; some are short and some are tall." Imam عليه السلام said: "The wall of the mosque of the Messenger of Allah سبحانه وتعالى, may Allah سبحانه وتعالى bless him and his family, at that time was the height of a man (qamah). And the one arm’s span and two arm’s spans were set so that no supererogatory prayer would be performed during the time of an obligatory prayer."