Which rūpa-kammaṭṭhāna did the Buddha practice when he was studying under Āḷāra Kālāma? On which meditation object did he practice the rūpa-jhānas in order to then transition to the arūpa-jhānas?
I am asking this because right before his enlightenment under the Bodhi tree, the Buddha recalled practicing ānāpānasati during his childhood—not during his time with Āḷāra Kālāma. Does this mean he was most likely using a kasiṇa to attain the rūpa-jhānas while training with Āḷāra Kālāma?