[Jacob] said, "Never will I send him with you until you give me a promise [i.e., oath] by Allāh that you will bring him [back] to me, unless you should be surrounded [i.e., overcome by enemies]." And when they had given their promise, he said, "Allāh, over what we say, is Entrusted."1
Translation: Saheeh International (English)
1 i.e., sufficient to witness and deal with the matter.
لَتَأۡتُنَّنِيthat surely you will bring him to melatatunnanī
بِهِۦٓthat surely you will bring him to mebihi
إِلَّآunlessillā
أَنthatan
يُحَاطَyou are surroundedyuḥāṭa
بِكُمۡۖyou are surroundedbikum
فَلَمَّآAnd whenfalammā
ءَاتَوۡهُthey had given himātawhu
مَوۡثِقَهُمۡtheir promisemawthiqahum
قَالَhe saidqāla
ٱللَّهُAllahl-lahu
عَلَىٰoverʿalā
مَاwhatmā
نَقُولُwe saynaqūlu
وَكِيلٞ(is) a Guardianwakīlun
Tafsir
Their father told them he would not send their brother with them until they had sworn by Allah that they would bring him back, unless destruction overwhelmed them all and none of them remained, and they were unable to do anything about it or to return. When they had given their father a firm promise in Allah's name, he said that Allah was a witness over what they said, and His witnessing was sufficient for them.