Brihat Parashara Hora Shastra
दशाब्दांघ्रिश्च पापानां शुभानां त्र्यंश एव हि । एकैकभे दशामानं विज्ञेयं द्विजसत्तम ॥ २१॥
The Dashas of the various Grahas have been specified above. In the case of malefic Grahas the Dasha span of one Nakshatr is ¼ of the Dasha of the Grah. It is 1/3rd in the case of benefics.
english translation
dazAbdAMghrizca pApAnAM zubhAnAM tryaMza eva hi । ekaikabhe dazAmAnaM vijJeyaM dvijasattama ॥ 21॥
hk transliteration by Sanscript