It would have been better if you had recited Sūrat al-A‘la, Sūrat Ash-Shams, or Sūrat al-Layl, for the old, the weak, and the needy pray behind you

The dwellings of Banu Salāmah, the clan of Mu‘ādh ibn Jabal al-Ansāri (may Allah be pleased with him) were located at the outskirts of Madīnah. Mu‘ādh (may Allah be pleased with him) was earnestly eager to do good deeds. He was keen to attend the prayer with the Prophet (may Allah's peace and blessings be upon him) due to his love for knowledge and desire to learn it.

He took a habit of offering the obligatory prayer in congregation with the Prophet (may Allah's peace and blessings be upon him). Then he would go to his people and lead them in the same prayer that he had just offered with the Prophet, may Allah's peace and blessings be upon him; in which case it was a voluntary prayer for him but an obligatory one for his people. Mu‘ādh did this with the knowledge of the Prophet (may Allah's peace and blessings be upon him) who approved this action of his.

However, Mu‘ādh prolonged the recitation of the Qur’an in the prayer one time. It is a matter of fact that the Islamic legislation is characterized by tolerance and ease without complicating things for people because this leads to negative consequences, among which is causing aversion to people.

When the Prophet (may Allah's peace and blessings be upon him) learned that Mu‘ādh prolonged the recitation of the Qur’an, he instructed him to shorten the recitation whenever he was leading the people in prayer. He gave him examples of the medium length Sūrahs of Al-Mufassal, such as Sūrat al-A‘la, Sūrat Ash-Shams, and Sūrat al-Layl, to recite in prayer given the fact that the elderly, the weak, and those who had urgent needs prayed behind him.

Since prolonging the prayer would cause hardship to such people, it was better to adopt a lenient attitude with them without burdening them with a lengthy prayer. However, if a Muslim is praying alone, he may prolong the prayer as he wishes.