صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب قيام المأمومين خلف الإمام وما فيه من السننChapters on Followers Standing Behind the Imam
(131) بَابُ ذِكْرِ الْبَيَانِ أَنَّ مُعَاذًا كَانَ يُصَلِّي مَعَ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَرِيضَةً لَا تَطَوُّعًا كَمَا ادَّعَى بَعْضُ الْعِرَاقِيِّينَ.131| Chapter clarifying that Mu‘adh prayed the obligatory prayer with the Prophet, not a voluntary prayer as claimed by some
Hadith 1635
It is narrated from Sayyiduna Jabir (may Allah be pleased with him) 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 prayer. Imam Abu Bakr (may Allah have mercy on him) says that I have already written this issue in detail. I have mentioned in this issue the ahadith of the Noble Prophet (peace be upon him) regarding the prayer of fear, that the Noble Prophet (peace be upon him) led one group in a voluntary prayer while they performed the obligatory prayer behind him. Thus, that prayer was voluntary for the Noble Prophet (peace be upon him) and obligatory for them.
