What is the basis for the argument that the minimum obligatory amount is equivalent to the Takbirat al-Ihram?

Primary text

That which is less than a Rak'ah necessitates the obligation of the second Rak'ah; therefore, the first obligatory unit must also be established, similar to the situation involving five Rak'ahs according to Malik. Furthermore, this act of commencement is one where the small and large quantities are equal, analogous to a traveler joining a resident worshipper (where the act of joining is established regardless of the remaining quantity).