# Origin of a problem in graph theory/planar geometry

Does anyone remember where does the following problem comes from:

Let $P_n$ be a set of $n$ points on the plane, and denote by $d$ the minimal distance between any two points of $P_n$ (i.e. $d=\min\{d(p_i,p_j)|p_i\neq p_j\in P_n\}$). Then there exists a subset $X \subset P_n$ such that for all $p_i,p_j\in X$, $d(p_i,p_j)>d$ and $|X|\geq\frac{n}{4}$

If I remember correctly, this was an open problem (which was solved quite easily using the four color theorem). I just can't remember who asked it (presumably Erdős, but i'm not sure) and when.