Sunni books mentioned that Imam Ali (AS) refused to give allegiance to Abu Bakr and Abu Bakr sent Umar with group of people to force Imam Ali on giving allegiance or burn his house on him and Fatima and Hasan and Husain.
(Tareekh Tabari (2: 443).
Mussannaf Ibn Abi Shaibah, (8:572)
Ansaab Al-Ashraaf by Balathori (1:586).
Al-Soyooti inMusnad Fatima , Page 36.
Ibn Qutaibah in Al-Imamah wal Siyasah (1:19) and (1:30).
Ibn Abd rabbih in Al-'iqd al-Fareed (1:78).
Ibn Abd al-Barr in Al-Estee'aab (3:975).
And many other Sunni books.
Those who claim that Imam Ali (AS)has given allegiance to Abu Bakr after six months need to prove their claim. There is no authentic narration but a narration from Aiysha Bint Abi Bakr claiming that. Everyone knows the stand of Aiysha against Imam Ali (AS).
We have in the books that after the demise of Lady Fatimah (AS), the Saqeefa people threatened to kill Imam Ali if he does not give allegiance and took from him an allegiance under threat. Obviously, even if this narration was right, such allegiance is invalid according to Islam because it was under threatening to kill him.
Imam Ali (AS) did not even pray with them in the Masjid nether Jumm'ah nor Jama'ah. (Siyar A'laam Al-Nubalaa' By Al-Dhahabi (9:284).