Siebs's law is a Proto-Indo-European (PIE) phonological rule named after the German linguist Theodor Siebs. According to this law, if an s-mobile is added to a root that starts with a voiced or aspirated stop, that stop is allophonically devoiced.
Compare:
PIE > Latin fragor,
but > PIE > Sanskrit sphūrjati.
Siebs proposed this law in the Zeitschrift für vergleichende Sprachforschung auf dem Gebiete der indogermanischen Sprachen, as Anlautstudien. Oswald Szemerényi rejected this rule, explaining that it is untenable and cites the contradiction present in Avestan zdī from PIE "be!" as counterproof. However, the PIE form is more accurately reconstructed as from (so not an s-mobile) and thus Siebs' law appears to demand that the sibilant and aspirated stop are both adjacent and tautosyllabic, something which is known to only occur in word-initial position in Proto-Indo-European anyway.