عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا جِئْتُمْ إِلَى الصَّلاَةِ وَنَحْنُ سُجُودٌ فَاسْجُدُوا وَلاَ تَعُدُّوهَا شَيْئًا وَمَنْ أَدْرَكَ الرَّكْعَةَ فَقَدْ أَدْرَكَ الصَّلاَةَ
Abu Hurairah reported that the Messenger of Allah ﷺ said, “When you come to pray and we are prostrating, you must join in prostration, but do not reckon this prostration as part of your Ṣalāh (i.e. a rak’ah). Whoever joins the imam in rukū’, he has been present at the prayer. [Abū Dāwūd]
Abdullah Ibn ʿUmar, said: “If you catch the Imam whilst bowing and bow (with him) before he raises his head, then you have caught the unit of the prayer (rak’ah). If he raises his head whilst you are bowing (and your hands have not yet reached your knees), then you have missed the unit of the prayer (rak’ah).” [al-Bayhaqī]
According to the aforementioned narration, the majority of scholars, including the ḥanafī school of thought conclude that if one enters the congregation and catches the imām whilst he is standing for recitation (qiyām) or in the bowing position (ruku’), then he has caught this unit of the prayer (rak’ah) even if he never recited anything. Catching the unit of the prayer means that he would not need to make up this particular unit after the final salām.
In another narration of a similar meaning, Rasūlullāh ﷺ said, “When one of you comes to the Ṣalāh and (finds) the Imam is in a position, then do as the Imam is doing.” [Tirmidhī]
In light of the above ḥādīth, we learn that if one enters the mosque and the imam is in Ṣalāh, he should join him no matter what position he is in, whether he is standing, bowing, prostrating or between the two prostrations.
Ḥāfiẓ Ibn Ḥajar ʿAsqalānī has said, “The above ḥādīth is quoted as evidence that it is mustaḥabb (desirable) to join the imam in whatever position he is found.” [Fatḥ-ul-Bārī]
Ibn Qudāmah has written:
“It is mustaḥabb (desirable) for the one who finds the imam in any position to follow him in that, even if it will not count as anything for him… Then he quoted some aḥādīth which are of a similar nature to that which we mentioned above… Then he said: This is what is to be done according to the scholars, who said: If a man comes and the imam is prostrating, let him prostrate, but that rak’ah (unit) will not count for him. Some of them said: perhaps by the time he raises his head, he will have been forgiven.” [al-Mughnī]
There are many people who, after entering the masjid and finding that the imām is prostrating or is between the two prostrations, they do not join him until he stands up for the second rak’ah (unit). Such a person has deprived himself of the benefit of prostration, as well as going against what is indicated by the ḥādīth quoted above.