Sahih Ibn Khuzaymah

Contrary to the statement of those Iraqi scholars who say that it is not permissible for one performing an obligatory prayer to follow one performing a supererogatory (nafl) prayer.

Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) narrates that Sayyiduna Mu'adh (may Allah be pleased with him) used to perform prayer with the Messenger of Allah (peace and blessings be upon him), then would return and lead his people in prayer, and he would recite the same prayer for them.

Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) narrates that Sayyiduna Mu'adh (may Allah be pleased with him) used to perform the 'Isha prayer with the Messenger of Allah (peace be upon him), then return and lead his companions in the ('Isha) prayer. One day, when he returned, he led them in prayer, and a young man from his people also began to pray behind him. When the recitation (of Sayyiduna Mu'adh) became lengthy for this young man, he (prayed alone) and left. He took hold of his camel's rein and went away. When Sayyiduna Mu'adh completed the prayer, he was informed of this. He said, "Indeed, this is hypocrisy; I will surely inform the Messenger of Allah (peace be upon him)." Therefore, Sayyiduna Mu'adh (may Allah be pleased with him) told the Messenger of Allah (peace be upon him) the story of the young man. The young man said, "O Messenger of Allah (peace be upon him), Sayyiduna Mu'adh stays with you for a long time, then returns and leads us in a lengthy prayer." Upon this, the Messenger of Allah (peace be upon him) said, "O Mu'adh, are you a cause of tribulation (fitnah)?" And he asked the young man, "O nephew, how do you pray?" He replied, "I recite Surah Al-Fatihah and ask Allah for His Paradise and seek refuge with Him from Hellfire, but I do not know your murmuring and that of Sayyiduna Mu'adh (i.e., which supplications you make)." The Messenger of Allah (peace be upon him) said, "Indeed, I and Mu'adh also murmur around these two (i.e., supplications for Paradise and seeking refuge from Hellfire)," or he gave a similar answer. The young man said, "But Mu'adh will soon know (whether I am a hypocrite or a true believer) when the people come (to the battlefield) and they know that the enemy has drawn near." Then, when the people descended into the field, the young man attained martyrdom. After that, the Prophet (peace be upon him) asked Sayyiduna Mu'adh, "What happened to the one who opposed me and you?" He replied, "O Messenger of Allah (peace be upon him), he proved his word true to Allah; he was martyred, while my statement turned out to be wrong (when I called him a hypocrite)."