Ibn Abbas (may Allah be pleased with them both) says: The word 'uqsimu' (I swear) on its own does not constitute an oath until the words 'uqsimu billah' (I swear by Allah) are uttered. In another statement it says: The word 'ashhadu' (I bear witness) also does not constitute an oath until the words 'ashhadu billah' (I bear witness by Allah) are spoken.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19887