"Let him prostrate twice while sitting." Then the Messenger of Allah, peace and blessings be upon him, turned and prostrated twice.

1204 - 'Amr ibn Rafi' narrated to us, Isma'il ibn 'Ulayyah narrated to us, from Hisham, Yahya narrated to me, 'Iyad narrated to me that he asked Abu Sa'id al-Khudri, saying: "One of us prays and does not know how much he has prayed." He said: "The Messenger of Allah, peace and blessings be upon him, said: 'When one of you prays and does not know how much he has prayed, let him prostrate twice while sitting.'"

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(1) Its chain of transmission is authentic. Al-A'mash: he is Sulayman ibn Mihran, and Ibrahim: he is Ibn Yazid al-Nakha'i, and 'Alqamah: he is Ibn Qays al-Nakha'i.
It was also recorded by Muslim (572) (94), (95), and (96), Abu Dawud (1021), and al-Nasa'i 3/66 via multiple paths from al-A'mash, with this chain of transmission. The narration of al-Nasa'i is abridged.
It is in "Musnad Ahmad" (4032).
It will come later via the path of al-Hakam from Ibrahim under number (1205), and via the path of Mansur from Ibrahim under numbers (1211) and (1212).
It was also recorded by Muslim (572) (92), and al-Nasa'i 3/32 and 33 via the path of al-Hasan ibn 'Ubayd Allah, from Ibrahim ibn Suwayd al-Nakha'i, from 'Alqamah, with it.
It was also recorded by Muslim (572) (93), and al-Nasa'i 3/33 via the path of 'Abd al-Rahman ibn al-Aswad ibn Yazid, from his father, from Ibn Mas'ud.
(2) A sound hadith, but this chain of transmission is weak due to the unknown status of 'Iyad ibn Hilal al-Ansari, and there is disagreement regarding his name as will come later. Hisham: he is al-Dastuwa'i, and Yahya: he is Ibn Abi Kathir.
It was also recorded by Abu Dawud (1029), al-Tirmidhi (398), and al-Nasa'i in "al-Kubra" (590) and (591) via the path of Yahya ibn Abi Kathir, with this chain of transmission. =