Translation:Shulchan Aruch/Orach Chaim/293
293: The Laws of the Evening Service on Motzei Shabbat. Contains 3 seifim.
1 We draw out the Maariv prayer in order to add holiness to the weekday.
2 One needs to be careful not to do work until three small stars will be seen; and they should not be scattered, but rather in a row. And if it is a cloudy day, one should wait until the uncertainty will go away from his heart.
3 One who is forced, such as one who needs to wait until dark at the techum for the sake of a mitzvah, could recite the prayers of Motzei Shabbos from Plag HaMinchah and onward and recite Havdalah immediately. However, he should not recite the blessing over the candle; and likewise, he is forbidden to perform work until the emergence of the stars. Gloss: And it is the custom to say "And He, the Compassionate One..." and "Bless..." with a protracted pleasantness so as to add from mundane to the holy. (Or Zarua).