According to Fajan's rule ionic character should increase down the group as the size of cation increase.
So it must be
$$\ce{LiH < NaH < KH < RbH < CsH}$$
However, the following two sources provide two different opinions. Which one is right?
Source 1
Explanation: As the size of the metal cation increases down the group, the lattice energy of the hydrides decreases down the group. Consequently, the reactivity of the alkali towards hydrogen decreases down the group.
iii. The ionic character of the alkali metal hydrides increases from Li to Cs.
Explanation: Since ionisation enthalpy of alkali metals decreases down the group, tendency to form cations as well as the ionic character of the hydrides increases.
Source 2
But it we are asked to compare the ionic character of hydrides (LiH, NaH, KH, RbH, CsH), we can not use the Fajan's rule. This is because, here the anion is very small ($\ce{H-}$) as compared to the cations (alkali metal cations). The ionic character is this case can thus be compared on the basis of Lattice energy.
Since lithium ion and the hydride ion are approximately of same size, the charge density is high between the two, thus the ionic bond so formed will be very strong and hence more will be the lattice energy.
On coming down the group, the difference in the size of cation and anion increases, thereby making the ionic bond weaker, thus reducing the lattice energy.
Also, lesser will be the lattice energy, lesser will be the ionic character.
Thus, down the group, the ionic character of the hydrides decreases.