الكافي- ط الاسلامية, volume 3, page 354

9. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from Ali ibn Hadid from Jamil certain persons of our people who have said the following: “Abu ‘Abdullah (a.s.), has said, ‘ One may not know whether he has performed three or four and his guess about both sides is equal. (For such case) he (the narrator) has said that he (the Imam) said, ‘If his guess is equal about three and four, he has the choice to perform one Rak’ah standing or two Rak’ah in a sitting position and four Sujud (prostrations) in a sitting position.’ He (the Imam) (a.s.), about a man who does not know if he has performed two or four Rak’ah and his guess about four is greater or about two Rak‘at, he (the Imam) said that he performs two Rak‘at and four Sujud (prostrations). He (the Imam) said, ‘If your guess about two Rak‘at and four is equal, then this and the case of doubt about whether it is three or four Rak‘at is not the same.”’

Translation: Muhammad Sarwar — attributed by Thaqalayn·The Arabic above is the original.