Ata (may Allah have mercy on him) says: When I say the word 'aqsamtu' (I swear), I do not give expiation until I say the words 'aqsamtu billah' (I swear by Allah).
A musnad hadith has also been narrated on this, but it is extremely weak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19886