(a) It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) came out to the Companions while they were waiting for Isha, and he (peace and blessings be upon him) said: "The people have prayed and gone to sleep while you are waiting for it; indeed, you have remained in prayer ever since you have been waiting for it. Were it not for my fear for the weakness of the weak and the old age of the elderly, I would have delayed this prayer until half of the night."
(b) Shu'bah (may Allah have mercy on him) says: I met him a second time and he said: or up to a third of the night. Khalid bin al-Harith (may Allah have mercy on him) reports from Shu'bah (may Allah have mercy on him): up to half of the night. It is narrated from Abu al-Minhal (may Allah have mercy on him): up to a third of the night.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1759