'Ali b. Abi Talib was in the mosque when he received the news of 'Uthman's murder. He soon left for home, where he was pressed by Companions visiting him to accept the pledge of allegiance.1 At first he refused, and then insisted that any pledge should be made in public in the mosque.2 Talha and al-Zubayr were of the first prominent Companions who pledged allegiance.3
'Ali personally abstained from putting pressure upon anyone to do homage. When Sa'd b. Abi Waqqas was brought, and asked to pledge of allegiance, he answered that he would not do so before the people had given their pledge, but assured 'Ali that he had nothing to fear from him. 'Ali gave orders to let him go.4 Then 'Abd Allah b. 'Umar was brought. He also said that he would pledge allegiance only after the people were united behind him. 'Ali asked him to provide a guarantor that he would not abscond; Ibn 'Umar refused. Now Malik b. al-Ashtar [al-Nakha'i] said to 'Ali, “This man is safe from your whip and sword. Let me deal with him.” 'Ali answered, “Leave him, I will be his guarantor. By God I have never known him other than ill-natured, as a child and as an adult.”
Ibn 'Umar's stand was, however, hostile to 'Ali. He left for Mecca to join the opposition.5 'Ali also invited Usama b. Zayd to pledge allegiance, but Usama, while assuring 'Ali that he was the dearest person to him, excused him on grounds of the commitment he had made to the Prophet (S.A.W.A.) never to fight anyone confessing, “There is no god but Allah”. When 'Ali asked Muhammad b. Maslama to pledge allegiance he, too, excused himself stating that the Prophet (S.A.W.A.) had ordered him, if there was conflict among the people, to break his sword and stay at home.6

Notes
1. See Nahj al-balagha, 1: 40 (At that moment the crowd frightened me. It advanced towards me from every side like the mane of a hyena so much so that al-Hasan and al-Husayn were being crushed and both of the ends of my shoulder garment were torn).
2. Tabari, Ta'rikh, 6: 3066.
3. Ibid, 3068.
4. Ibid, 3068.
5. Baladhuri, Ansab, 3: 8
6. Ibid, 3: 9.