I say: Ibn Umar’s practice regarding vows to commit a sin was to withhold judgment, directing neither their fulfillment nor their abandonment. Thus, when asked about someone who had vowed to fast on the day of Eid, he said:
“Allah has commanded the fulfillment of vows, and the Messenger of Allah, peace and blessings be upon him, has forbidden fasting on this day.”
This was because he saw a conflict between two proofs: the command and the prohibition. It had not become clear to him that the command to fulfill vows is restricted to acts of obedience to Allah. Hence Mālik transmitted in his Muwatta’ the hadith subsequently recorded by al-Bukhārī, from Ā’isha, that the Messenger of Allah, peace and blessings be upon him, said:
“Whoever vows to obey Allah, let him obey Him; and whoever vows to disobey Allah, let him not disobey Him.”
Moreover, the Quran contains no unrestricted command to fulfill vows explicitly using the word “vow.” His words, “They fulfill their vows,” are a statement of fact and praise, while His words, “Let them fulfill their vows,” concern a specific case. Allah has, however, commanded the fulfillment of covenants and contracts, and vows fall within that category. This, and Allah knows best, is what the two men meant when they said, “Allah has commanded the fulfillment of vows.”
Such is the position of someone who regards covenants and contracts as requiring fulfillment unconditionally, without considering their subject matter. This often happens to some scrupulously pious people, as it did to Ibn Umar: they refrain from breaking many covenants and contracts that contravene the Sharia, yet their scrupulousness also makes them refrain from contravening the Sharia, leaving them bewildered.
As for Ibn Abbās, two opinions are reported from him on this issue. The first is the view just mentioned. The second is that the person must slaughter a ram. This is also one of the two opinions reported from Ahmad, and the view of Abū Hanīfa and others. This is what accords with the Sharia, rather than appealing to the story of Abd al-Muttalib, for the practices of the people of pre-Islamic ignorance cannot be cited as proof at all unless Islam has endorsed them.