Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) says that Sayyiduna Mu'adh (may Allah be pleased with him) used to pray the Isha prayer with the Prophet (peace and blessings be upon him), then come and lead his people in prayer; for him it was voluntary, and for those following him it was obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5105