Application note Basic Calculation of a Buck Converter’s Power Stage (Rev. B) says:
6 Input Capacitor Selection
The minimum value for the input capacitor is normally given in the data sheet. This minimum value is
necessary to stabilize the input voltage due to the peak current requirement of a switching power supply. The best practice is to use low-equivalent series resistance (ESR) ceramic capacitors. The dielectric material must be X5R or better. Otherwise, the capacitor loses much of its capacitance due to dc bias or temperature. The value can be increased if the input voltage is noisy.
The data sheet says:
18.104.22.168.2 Input Capacitor
For most applications, 10 μF is sufficient and is recommended, though a larger value reduces input current ripple further. The input capacitor buffers the input voltage for transient events and also decouples the converter from the supply. A low ESR multilayer ceramic capacitor is recommended for best filtering and should be placed between VIN and PGND as close as possible to those pins.