The tisarana is three short lines, repeated three times: I go to the Buddha for refuge, I go to the Dhamma for refuge, I go to the Sangha for refuge. Its grammar is the point. Where the Nicene Creed says the Son is homoousios with the Father, and the Shahada says there is no god but God, the tisarana asserts nothing about the nature of the Buddha, the truth of the teaching or the character of the community. It commits the speaker to a direction rather than to a proposition, which is why Buddhist writers resist calling it a creed at all and speak instead of taking refuge, going for refuge, or simply the refuges.
The formula is recorded in the Pali Canon, at Khuddakapatha 1 among other places, and its use at ordination is old enough that the Vinaya, the monastic rule, assumes it as the entry rite into the monastic community as well as into lay identity. A candidate for ordination recites it before the assembly, and a layperson recites the same words to mark taking up Buddhist identity for the first time or reaffirming it. Because it has no author and no synod that promulgated it, its authority is entirely a matter of unbroken use: every school that calls itself Buddhist, Theravada, Mahayana or Vajrayana, keeps some version of these three lines, usually in Pali or in a local rendering of the Sanskrit buddham sharanam gacchami, and keeps them in the same triple form. Repetition itself carries theological weight here. Saying the formula three times is treated in commentarial literature as the minimum for the going for refuge to be complete, so the counting is not decorative but constitutive of the act.