Distortion of facts by Banu Ummayah in their reign.

Ahmad bin Hanbal (Died 241 AH) narrated : 

حدثنا عبد الله قال: حدثني أبي، قثنا عبد الرزاق قال: أنا معمر قال: سألت الزهري: من كان كاتب الكتاب يوم الحديبية؟ فضحك وقال: هو علي، ولو سألت هؤلاء قالوا: عثمان، يعني بني أمية.

Narrated by Ma'mar who said that he asked Zuhri :

Who wrote the treaty on the day of Hudaibiyyah ? He laughed and said : It was Ali (Ibn Abi Talib) and If you will ask them meaning Banu Ummayah, They will tell you it was Uthman.

Investigator of the book "Wasiullah Abbas" has decalred the chain of narration to be Authentic (Sahih).

Kitab Fadhail u Sahabah - Ahmad bin Hanbal // Volume 2 // Page 59 // Number 1002 // Edn. Dar Ibn Jawzi Riyadh Saudi Arabia.

Same can be seen in Urdu translated edition.

Fadhail u Sahabah - Ahmad bin Hanbal (Urdu) // Page 330 // Number 1002 // Edn. Dar ul Al-Ilm Mumbai India. 

Abdur Razaq San'ani (Died 211 AH) has also narrated it in his book. 

Musanaf - Abdur Razaq // Volume 5 // Page 22 // Number 10458 // Edn. Dar Al-Tasil Riyadh Saudi Arabia.

So you can see how Banu Ummayah during their power used to distort the historical facts to belittle the status of Imam Ali (as).