Kindly mentioned some Hadiths with authenticity and reference for Isal thawab, especially regarding dhikr like the first kalimah or other adhkar which are recited for Isal thawab.
In principle, Isal thawab can be done with any nafl ‘ibadah, be it nafl Salah, nafl fast, nafl haj or even dhikr or Quran recital.
See a detailed article, with the proofs you’ve requested here.
Kindly refer to a reliable Mufti/Darul ifta for more.
And Allah Ta’ala Knows best,
Answered by: Moulana Muhammad Abasoomar