Assalam Alaykum Ustadh. In tashahud we say 'Peace be upon you, O Prophet' Would this be invoking upon the dead ? Ibn Mas'ub said they used to say this when the prophet was alive but then changed wording to As-Salâmu 'alan-Nabî after his death. Which wording is correct?

Ustadh AbdulAziz al-Haqqan

No it is not and this is what the ahadeeth report so do not fall into waswas.