Simple method to calculate output impedance of source follower
With R_s = 0, an M1 impedance contribution and an C_gs impedance are connected in parallel; the output impedance is
$$ 
Z_0 = {1\over{g_m}} || {1\over{s·C_{gs}}}
$$
Neglecting C_ds, an R_s contribution is in series, only it should be scaled down. Common drain stage is a non-inverting current amplifier. Given a common drain stage's AC current gain
$$ 
A_{cd} = {1 + g_m\over{s·C_{gs}}}
$$
we can calculate a current thru R_s and a scaled-down R_s contribution R_sScaled into the output impedance
$$ 
R_{sScaled} = {R_s\over{A_{cd}}} = {R_s\over{1 + g_m\over{s·C_{gs}}}} = {R_s·s·C_{gs}\over{s·C_{gs} + g_m}}
$$
The total output impedance is
$$ 
Z = Z_0 + R_{sScaled}
$$
$$
Z = {1\over{s·C_{gs} + g_m}} + {R_s·s·C_{gs}\over{s·C_{gs} + g_m}} = {{1+R_s·s·C_{gs}}\over{s·C_{gs} + g_m}}
$$
The only problem with this derivation is that it is only trustworthy for those who knows how to get the result with brute force method. For someone who has never analyzed an AC small-signal equivalent circuit of common drain configuration, it may be difficult to see a Thevenin behind the above equations.