When the Quraish lost hope in Abu Taalib supporting them, they began to show open opposition and enmity for the Muslims. They started inflicting different punishments on the weak and vulnerable Muslims of the various tribes.
Abu Taalib then called on the Banu Haashim and Banu Muttalib to support and assist Rasulullah (sallallahu ‘alaihi wasallam). Upon the call of Abu Taalib, all the members of the Banu Haashim and Banu Muttalib responded and promised their assistance. From the Banu Haashim, only Abu Lahab joined the enemy against Rasulullah (sallallahu ‘alaihi wasallam).
(Extracted from Seeratul Mustafa 1/178-179)