The imamate of all the following is acceptable: a discerning boy, a blind person, a standing person for those who are sitting, a sitting person for those who are standing, a person praying fard for people who are praying nafl, a person praying nafl for people who are praying fard. Likewise, a person who has performed ablution can be imam for people who have performed tayammum, as can be a person who has performed tayammum for people who have performed ablution, a traveller for the resident, a resident for the travellers, and a less qualified person for people who are more qualified.
‘Amr ibn Salamah led his people in salah while he was six or seven years old. The Messenger of Allah sallallahu alehi wasallam twice appointed Ibn Umm Maktum, a blind man, to lead the people of Medinah in prayer. The Messenger of Allah, during his last illness, prayed behind Abu Bakr in a sitting position. And he prayed in his house in a sitting position while those behind him were standing. He pointed to them to sit and when he had finished the prayer he said: “The imam has been appointed to be followed. If he goes into ruku’, then make ruku’. When he raises his head, raise your head. If he prays sitting, then pray sitting behind him.”
Mu’azh would pray ‘isha with the Prophet sallallahu alehi wasallam and then return to his people and lead them in the same prayer, it being nafl for him and fard for the others.
Muhjan ibn al-Adra’ reports: “I came to the Messenger of Allah in the mosque and they prayed and I did not. He said to me: ‘Why didn’t you pray?’ I said: ‘O Messenger of Allah, I prayed in my place and then came here.’ He then said: ‘When you come [to the mosque], pray with them and make it supererogatory.”‘
The Messenger of Allah saw a man praying by himself and said: “Who will give charity to this person by praying with him?” ‘Amr ibn al-‘Aas led others in prayer when he had made tayammum only and the Prophet approved of it.
The Prophet sallallahu alehi wasallam, after the conquest of Makkah, led the people in prayer by praying two rak’at (except for maghrib) and said: “O people of Makkah, stand and pray the last two rak’at as we are travellers.”
If a traveler prays behind a resident, he must complete the whole four rak’at even if he only prayed part of a rak’ah behind the resident imam. Ibn ‘Abbas was asked: “Why is the traveller to pray two rak’at if he prays by himself and four rak’at if he prays behind a resident?” He answered: “That is the sunnah.” In another version, Musa ibn Salamah said to him: “If we pray with you, we pray four rak’at otherwise we pray two?” He told him: “That is the sunnah of Abu al-Qasim [the Prophet].” This is related by Ahmad.