Sheikh Abu al-Faraj (15) said in "al-Mubhij": He should treat the prayer he has entered into as the second [prayer] as a completion for the first, so he builds one upon the other, and the presence of the taslim is as if it were non-existent, because it is an act of forgetfulness for which he is excused. This applies regardless of whether what he entered into was a supererogatory prayer or an obligatory one. al-Hasan and Hammad ibn Abi Sulayman said: [If he enters into a supererogatory prayer, the obligatory prayer is invalidated. Malik said: I prefer that he restarts it. It is narrated from Ahmad, may Allah have mercy on him, similar to the saying of al-Hasan; for he said] (16), in the narration of Abu al-Harith, if he prays two rak'ahs of Maghrib and performs the taslim and then enters into a supererogatory prayer: It is in the status of speech, he must restart the prayer. Our position is that he performed an act of the same genus as the prayer out of forgetfulness, so it is not invalidated, just as if he had added a fifth rak'ah. As for [completing the first with the second] (17), it is not valid; because he has exited [the first by the taslim, and the intention of exiting it while he did not intend it (the prayer)] (18), and the intention for something else does not suffice for its intention, as in the state of the beginning.
215 - Issue: He said: (And whoever is an Imam and doubts, not knowing how much he has prayed? He shall exercise personal effort (taharra), then build upon his strongest assumption, then perform the prostration after the taslim, as narrated (1) by 'Abd Allah ibn Mas'ud, from the Prophet (may Allah bless him and grant him peace)).
His statement "upon his strongest assumption" means what he deems most likely to be what he has prayed. This is specifically for the Imam. Another narration is reported from Ahmad, may Allah have mercy on him: That he builds upon [his predominant probability (ghalib al-zann); whether he is an Imam or praying alone. He said, in the narration of al-Athram] (2): There is a difference between personal effort (taharri) and certainty. As for
(15) He is 'Abd al-Wahid ibn Muhammad al-Shirazi al-Maqdisi, one of the students of Abu Ya'la, who died in the year 486 AH. See: Mafatih al-Fiqh al-Hanbali 2/71, 72. (16) In M: "Regarding one who performs the taslim before completing the obligatory prayer and enters into a supererogatory prayer: it invalidates the obligatory prayer. Malik said: I prefer that he restarts it. Ahmad designated this, saying". (17) In M: "Building the second upon the first". (18) In M: "From the first and did not intend it after that". (1) In M there is an addition: "from". (2) Omitted from M. It includes: "certainty and he prostrates before the taslim..." up to his saying: "not performing that which he doubted in". And its place will come in the original text.
