Brihat Parashara Hora Shastra
दशा वर्षाणि राशीनामेकैकस्य दशामितिः । क्रमाच्चक्रस्थितानाञ्च विज्ञातव्या द्विजोत्तम ॥ ५१॥
If the birth is during Sandhya, the Dasha will begin from the Rāśi of the second Bhava. The Dasha of each Rāśi is 10 years. As it is the Dasha system of the 12 Rāśis in the Zodiac, it has been named, as Chakr Dasha.
english translation
dazA varSANi rAzInAmekaikasya dazAmitiH । kramAccakrasthitAnAJca vijJAtavyA dvijottama ॥ 51॥
hk transliteration by Sanscript