Answer
(Fatwa: 1187/1193/N=1433)
If anyone offers the two rakah nafl after witr or any other nafl salah in sitting position without a Shariah excuse, he shall get only half reward. The holy Prophet Muhammad (صلى الله عليه وسلم) was awarded full reward while offering it in sitting position, it was his exception. كذا فى الدر والرد (كتاب الصلاة باب الوتر والنوافل 2:484، 485 ط مكتبة زكريا ديوبند).
Hence offering the two rakahs after witr in sitting position cannot be termed as sunnah for the Ummah. Yes, if anyone offers these two rakahs purely with the emotion of following the sunnah while sitting then too as per the rule he shall get only half reward but in this case he shall receive the reward of following the holy Prophet Muhammad (صلى الله عليه وسلم). It might happen that this reward along with the reward of salah becomes equal to the reward which one gets while offering salah standing or more than it. Imam Gangohi Rahmatullah has said so. (Tohfatul Almai 2:327, 328)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband