It is narrated from Sayyiduna Abu Sa'id (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) delayed the Isha prayer until close to half of the night, then he came out and he (peace and blessings be upon him) led the prayer and said: "You have remained in prayer as long as you were waiting for it; and were it not for my concern for the old age of the elderly and the weakness of the weak" (the narrator says: I think he (peace and blessings be upon him) also said) "and the need of the one who has work to do, I would certainly have delayed this prayer until half of the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1758