It was narrated from Ibn 'Abbas that the Messenger of Allah (ﷺ) said:
'When anyone of you swears an oath, let him not say: 'What Allah wills and what you will.' Rather let him say: 'What Allah wills and then what you will.'
English reference: Vol. 3, Book 11, Hadith 2117
Arabic reference: Book 11, Hadith 2197