From Du Fu's (
杜甫) Tang poem 《
春夜喜雨》 (Delighting in Rain on a Spring Night):
好雨知时节,
当春乃发生。
随风潜入夜,
润物细无声 — 'Good rain knows its season; when spring comes, it brings life. It slips in with the wind at night, moistening things fine and soundless.' Now used figuratively for any subtle, formative influence.