Salam alaikum. Once we already posted information where shia scholar Shareef al-Murtada stated that majority of Ali’s followers befriended Abu bakr and Umar (may Allah be pleased with all of them) and accepted their Imamate.
Now I’d like to show you another quote from other known shia scholar at-Tabarsi.
In his book “Khatimatul Mustadrak” (5/18) he said:
ومن هنا يعلم: ان الذين قتلوا مع امير المؤمنين (عليه السلام) في الحروب الثلاثة كانوا شهداء وفيهم كثير ممن كانوا يتولونهما
“and from this it is known: that those who fought along with commander of faithful (alaihi salam) in 3 battles (Jamal;Nahravan; Siffin) and became a martyrs, and between them were a lot of (people) who befriended them”.
And in the footnote written:
أي ممن كانوا يتولون الخلفيتن: الاول، والثاني
Mean those who befriended two caliphs: First and Second.