Hisham bin Urwah narrates from his father that Aishah (may Allah be pleased with her) told him that she never saw the Prophet (peace and blessings be upon him) pray the night prayer sitting until he (peace and blessings be upon him) had grown old. Then he (peace and blessings be upon him) would recite sitting, and when he intended to bow, he would stand up, then recite thirty or forty verses, and then bow.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4589
Abu Salamah bin Abdur-Rahman narrates from Sayyidah Aishah (may Allah be pleased with her) that he (peace and blessings be upon him) would pray sitting and also recite sitting, and when thirty or forty verses of his (peace and blessings be upon him) recitation remained, he (peace and blessings be upon him) would stand up. Then he (peace and blessings be upon him) would recite standing and bow, and then prostrate. Then he would do the same in the second rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4590
Amrah narrates from Sayyidah Aishah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) would recite while sitting, and when he (peace and blessings be upon him) intended to bow, he would stand for as long as it takes a person to recite forty verses.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4591