Brihat Jataka
होरानवांशप्रतिमं विलग्नं लग्नाद्रविर्यावति च दृकाणे ॥ तस्माद्वदेत्तावति वा विलग्नं प्रष्टु: प्रसूताविति शास्त्र माह ॥ २६-७ ॥
It is said in the text, the lagna is equal to the navamsha lagna or one should declare the lagna of the querent when born as far as the Sun is from the lagna in the drekkana.
english translation
horAnavAMzapratimaM vilagnaM lagnAdraviryAvati ca dRkANe ॥ tasmAdvadettAvati vA vilagnaM praSTu: prasUtAviti zAstra mAha ॥ 26-7 ॥
hk transliteration by Sanscript