Surya Siddhanta
दूरस्थितः स्वशीघ्रोच्चाद् ग्रहः शिथिलरश्मिभिः। सव्येतराकृष्ततनुर् भवेत् वक्रगतिस् तदा॥
When a planet is at a great distance (more than 3 signs) from its Śíghrochcha and (therefore) its body is attracted by the loose reins (borne by the Śíghrochcha,) to its left or right, then the planet's motion becomes retrograde.
english translation
dUrasthitaH svazIghroccAd grahaH zithilarazmibhiH। savyetarAkRStatanur bhavet vakragatis tadA॥
hk transliteration by Sanscript