The view of Ata' bin Abi Rabah (may Allah have mercy on him), and of those Companions (may Allah be pleased with them) who said the like of his view, has already passed: that this is an oath, and its expiation is the same as the expiation of an oath.
Imam al-Shafi'i (may Allah have mercy on him) said: "Someone other than him said: He gives away in charity everything he owns, though he also said: He keeps back the amount that suffices for his sustenance, and when he becomes well-off, he gives away in charity what he had kept back.
Another (scholar) held that he gives a third of his wealth in charity, and another that he gives (the amount of) zakah in charity."
Al-Shaykh (Imam al-Bayhaqi) (may Allah have mercy on him) said: "As for the first view, it is reported from some scholars of Iraq; as for the second, it is the view of Imam Malik (may Allah have mercy on him), and some of those who follow his view have argued from the following narration."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / Q20050
Some of the sons of al-Sa'ib bin Abi Lubabah (may Allah have mercy on him) said: When Sayyiduna Abu Lubabah (may Allah be pleased with him) tied himself to the pillar and Allah Almighty accepted his repentance, he said: "O Messenger of Allah, I want to leave my house in which the sin occurred, live in your neighbourhood (peace and blessings be upon him), and spend all my wealth in the way of Allah." He (peace and blessings be upon him) said: "Give a third of your wealth in charity; that will suffice."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20050
Ibn Ka'b bin Malik (may Allah have mercy on him) narrates from his father (Sayyiduna Ka'b bin Malik, may Allah be pleased with him) that he, or Abu Lubabah (may Allah be pleased with him), or whomever Allah willed, said to the Prophet (peace and blessings be upon him): "Part of my repentance is that I leave the house of my people in which I committed the sin, and that I part with all my property as charity." He (peace and blessings be upon him) said: "Give only a third of your wealth in charity; that will suffice for you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20051
Ibn Ka'b bin Malik (may Allah have mercy on him) said: It was Abu Lubabah (may Allah be pleased with him), and he mentioned the same meaning. Abu Dawud (may Allah have mercy on him) said: The story is about Abu Lubabah (may Allah be pleased with him).
Al-Shaykh (may Allah have mercy on him) said: It is with this wording in the story of Abu Lubabah; as for what he (peace and blessings be upon him) said to Ka'b bin Malik, it was not fixed at a third.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20052
Abdullah bin Ka'b bin Malik (may Allah be pleased with him) narrates from his father that he said to the Messenger of Allah (peace and blessings be upon him), when his repentance was accepted: "O Messenger of Allah, I want to give my wealth in charity." The Messenger of Allah (peace and blessings be upon him) said: "Keep some of your wealth; that is better for you."
This hadith is sahih (authentic), while the first one is disputed in its chain, is not established as connected (mawsul), and it is not valid to use it as evidence in this matter. Abu Lubabah (may Allah be pleased with him) wanted to give his wealth in the way of Allah out of gratitude to Allah, so just as he (peace and blessings be upon him) had stopped Ka'b bin Malik (may Allah be pleased with him) from giving all his wealth, he stopped Abu Lubabah (may Allah be pleased with him) as well. It has not reached us that he made a vow or swore an oath.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20053
As for the third view, it is in this narration: Uthman bin Abi Hadir (may Allah have mercy on him) said: A woman from the family of Dhu Asbah swore, saying: "Her wealth is in the way of Allah and her slave girl is free if he does not do such and such" — something her husband disliked — and her husband swore that he would not do it. Sayyiduna Ibn Abbas and Sayyiduna Ibn Umar (may Allah be pleased with them) were asked about this, and they said: "The slave girl is free, but the zakah of her wealth is to be paid."
Thus it is in this narration; and we have narrated from Sayyiduna Ibn Abbas and Sayyiduna Ibn Umar (may Allah be pleased with them) what indicates that expiation is permissible, and Allah knows best. Another view on this has also been narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20054
Abu al-Juwayriyah reports that he heard Sayyiduna Ibn Abbas (may Allah be pleased with them both) (asked) about a man who owed a hundred camels if he spoke to his brother: (he said) that he should sacrifice thirty camels and speak to his brother.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20055