The statement regarding the command to base it on investigation and inquiry when his heart inclines more towards a certain number, and he predominantly thinks that the number his heart inclines towards is the number of prayers he has performed.

Sayyiduna Abdullah bin Mas’ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings of Allah be upon him) led us in prayer, and in that prayer he either added something or omitted something. Then the Messenger of Allah (peace and blessings of Allah be upon him) turned his blessed face towards us. We said,“O Messenger of Allah, has there been some change in the prayer?”The Messenger of Allah (peace and blessings of Allah be upon him) asked,“What (change) is that?”So we informed the Messenger of Allah (peace and blessings of Allah be upon him) about what he had done. Therefore, the Messenger of Allah (peace and blessings of Allah be upon him) bent his foot, faced the qiblah, and performed prostrations. Then, turning towards us, he said:“If there had been any change in the prayer, I would have informed you. But I am a human being; I forget just as you forget. So if I forget, remind me. And if any one of you is uncertain about his prayer (whether he has added or omitted), he should reflect on what is most likely correct, then complete his prayer accordingly, then say the salam and perform two prostrations.”This is the narration of Abū Mūsā from ‘Abdur Rahman. Abū Mūsā says that Ibn Mahdi said: I asked Imam Sufyan about this, and he said, I heard this narration from Mansur, but I do not remember it. Ahmad bin ‘Abdah, in his narration, did not mention the words“al-taharrī”(investigation and seeking certainty), and said: If any one of you forgets in his prayer and does not know how much he has prayed, then after saying the salam, he should perform two prostrations of forgetfulness. Imam Abu Bakr (may Allah have mercy on him) says: This hadith is evidence that when the praying person completes his prayer based on investigation and seeking certainty, he should perform the two prostrations of forgetfulness after the salam, and this is my position. And when he bases it on the minimum number, then he should perform the two prostrations of forgetfulness before the salam, as is in the narration of Sayyiduna Sa’id Khudri (may Allah be pleased with him). In my view, it is not correct or permissible to reject one narration with another; rather, it is obligatory to act upon each narration in its proper place.“Al-taharrī”(investigation and seeking certainty) is when the heart of the praying person inclines more towards one number, while the issue of basing on the minimum number is different from the issue of investigation. Therefore, it is obligatory to act upon both narrations as they have been stated.