Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with the same chain of narration (it is narrated from Sayyida Aisha, may Allah be pleased with her) that she said she never saw the Messenger of Allah, peace and blessings be upon him, performing the night prayer sitting down, until when he became old, he would recite while sitting, then when he intended to bow (for ruku‘), he would stand and recite about thirty or forty verses, then he would bow.