It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) (one night) delayed the Isha prayer until half the night, then came out and led the people in prayer and said: "As long as you are waiting for the prayer, that time is counted as being in prayer."
"Were it not for (concern for) the elderly and the weak, I would have delayed this prayer until half the night." The narrator said: Perhaps he (peace and blessings be upon him) also mentioned those with needs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2120