It is narrated from Jabir bin Abdullah Ansari (may Allah be pleased with him) that Mu'adh bin Jabal (may Allah be pleased with him) used to perform the (obligatory) 'Isha prayer with the Prophet (peace be upon him), then he would go to his people and lead them in the 'Isha prayer, and for him, this would be a voluntary prayer.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 305
Hadith Takhrijاخرجہ ابوداؤد، الصلاة، باب امامة من صلی بقوم وقد صلی تلك الصلاۃ: 599، واحمد: 3/302، وصححہ ابن خزیمہ: 1633، 1634۔