Bismillahi Al-Rahmani Al-Rahim
I have noticed that the Salafis pray eight rak'ats for tarawih.
Is there an opinion in the Hanbali madhab (even if weak) which agrees with the view of the Salafies?
Who first started this( praying 8 rakats)?
I have yet to find an opinion in the Hanbali madhhab that says that the sunnah for tarawih is 8 rak`at. What it does say is that the sunnah for tarawih is 20 rak`at, and that the sunnah for tahajjud is 8. In any case, you can pray as many supererogatory prayers as you want. Please see the previous messages such as "A Bit About Tarawih" for more information on the topic.
I really have no idea who started to call people to eight rak`at. It's not even mentioned in Manar Al-Sabil which was written at most 90 years ago. Looking through the Zuhair Shawish' summaries of what the late Nasr Al-Din Al-Albani had to say, he agreed that the following hadith mentioned in Musnad Imam Ahmad (21411) Al-Tirmidhi (810) is authentic:
And Allah knows best.
wa al-salamu `alaykum