Many siddurim in my possession indicate one who davens minchah after sunset omits tachanun. What are the sources behind this ?
Shulchan Aruch (Orach Chaim 131:3) writes:
אין נפילת אפים בלילה...:
There is no "Nefilat Apayim" [what we consider to be "Tachanun"] at night...
Mishnah Berurah there (note 16, based on Beit Yosef) argues that this ruling is based on kabbalistic reasons. There is a general discussion as to when halachic night begins; while Mishnah Berurah (note 17) rules that during the period of "Bein Hashmashot" one is still permitted to say Tachanun (based on Taz there), Kaf Hachaim (note 51) makes a very strong argument against it based on various sources, and primarily the Magen Avraham there (which Mishnah Berurah cites, but must interpret differently).