Answered by Ustadh Sufyan Qufi
It was narrated that Ibn Mas’ud (may Allah be pleased with him) said: “Whoever hears the muezzin and does not respond with no excuse, his prayer is not valid.”
Does this mean that the prayer is not valid if it is done alone?
In the name of Allah, Most Compassionate, Most Merciful,
I pray this finds you in the best of states.
The Prophet – peace be upon him – is reported as having said that “Praying in congregation is twenty-seven times better than praying alone.” [Bukhari; Muslim]
According to the Hanafi school, it is not obligatory but it is an emphasized sunna to pray in a congregation for men who have no valid excuse. [Taqrirat Ar-Rafi‘i on Hashiyat Ibn Abidin]
To get used to missing the congregational prayer is sinful without excuse. [Tahtawi/Shurunbulali, Hashiyat Maraqi al-Falah]
The sunna is fulfilled even by praying in a congregation outside of the mosque, even though it is better to perform it in the mosque. [Tahtawi/Shurunbulali, Hashiyat Maraqi al-Falah]
Thus a prayer prayed alone even without excuse will remain valid.
The Reason for This Ruling
The reason for this is that it is clearly understood from the Quran by the jurists of the Hanafi school that a prayer prayed alone even without excuse remains valid as the only obligatory actions listed in it are the standing, the recitation, the bowing, and the prostration. Claiming that prayer needs more than that to be valid entails amending the book of Allah, Most High, without a valid reason.
As for the last sitting for the extent of the Tashahud being obligatory despite being established through hadiths it is because these hadiths have been established very strongly and clarify what has been left undetailed regarding the validity of the prayer in the Quran. Thus these hadiths are practically applied by the jurists as if it has the strength of Quranic proof.
[Marghinani, Hidaya; Laknawi, Hashiya on the Hidaya]
As for the hadiths that may indicate that praying in congregation is obligatory (fard) and that praying alone without an excuse is invalid they are understood in light of what has been affirmed unequivocally by the Quran above which is that a prayer prayed alone without an excuse is defective but remain nonetheless valid.
And Allah knows best.
[Ustadh] Sufyan Qufi
Checked and Approved by Shaykh Faraz Rabbani