ع ث رʿ-th-r
(ن) عَثَرَ يَعْثُرُ عِثَارًاʿathara yaʿthuru ʿithāran: to stumble; to come upon some matter unintentionally (intransitive; it becomes transitive with the preposition عَليٰʿalyā). ﴿فَاِنْ عُثِرَ عَلٰٓى اَنَّهُمَا اسْتَحَقَّآ اِثْمًا﴾ (Al-Māʾidah 5:107)
Form IV (ifʿāl) اَعْثَرَ يُعْثِرُ اِعْثَارًاaʿthara yuʿthiru iʿthāran: to make someone aware of some matter. ﴿وَكَذٰلِكَ اَعْثَرْنَا عَلَيْهِمْ﴾ (Al-Kahf 18:21)