Musnad Ahmad 1153:249

Yahya narrated to us, from Hisham, who said: My father informed me, from Aisha, who said: "I did not see the Prophet, peace and blessings of Allah be upon him, reciting in any of the night prayers while sitting, until when he became old, he would recite while sitting. Until when there remained of the surah thirty or forty verses, he would stand and recite them, then he would bow."