Waki' bin al-Jarrah وكيع بن الجراح · The Chapters on Expiation

"When the Prophet (ﷺ) took an oath he would say: 'By the One in Whose Hand is the soul of Muhammad."'

"On the Day of the conquest of Makkah, he came with his father and he said: 'O Messenger of Allah, give my father a share of Hijrah.' He said: 'There is no Hijrah.' Then he went away and entered upon 'Abbas and said: 'Do you know who I am?' He said: 'Yes.' Then 'Abbas went out, wearing a shirt and no upper wrap, and said: 'O Messenger of Allah, do you know so-and-so with whom we have friendly ties? He…

"Your oath is as your companion understands it to be."

"Vows do not bring the son of Adam anything unless it has been decreed for him. But he is dominated by Divine preordainment, and will get what is decreed for him. And (vows) are a means of making the miser give something, so what he desires becomes obtainable for him, which was not obtainable before his vow. And Allah says: 'Spend, I will spend on you.'

"Whoever makes a vow and does not state it specifically, the expiation (for such a vow) is the expiation for breaking an oath. Whoever makes a vow and is not able to fulfill it, the expiation for that is the expiation for breaking an oath. Whoever makes a vow and is able to fulfill it, let him do so."

Sa'd bin 'Ubadah asked the Messenger of Allah (ﷺ) about a vow which his mother had made, but she had died without fulfilling it. The Messenger of Allah (ﷺ) said: "Fulfill it on her behalf ."

'Uqbah bin 'Amir told him, that his sister vowed to walk, barefoot and bareheaded, and he mentioned that to the Messenger of Allah (ﷺ) He said: "Order her to ride and to cover her head, and to fast for three days."