The Messenger of Allah ﷺ said, "When one of you rises from the second rakah but has not yet stood fully upright, let him sit back down. But if he has stood fully upright, let him not sit, and let him perform the two prostrations of forgetfulness."
حدثنا محمد بن يحيى حدثنا محمد بن يوسف حدثنا سفيان عن جابر عن المغيرة بن شبيل عن قيس بن أبي حازم
قال رسول الله ﷺ إذا قام أحدكم من الركعتين فلم يستتم قائما فليجلس فإذا استتم قائما فلا يجلس ويسجد سجدتي السهو
[Its chain is extremely daif]Abu Dawud transmitted it in his chapter on Prayer, in the section on one who forgets to recite the tashahhud while sitting, hadith no. 1036, from the hadith of Sufyan Thawri through this same chain, and Ibn Munzir declared part of it daif.Jabir Jafi, his status was already discussed at hadith no. 356, and Ibrahim bin Tahman and Qays bin Rabi followed him in transmitting it.