Al-A'mash (may Allah have mercy on him) said: It reached me from Ibn Abbas (may Allah be pleased with them both) that he said: Had he pressed me (with excuses), I would have ordered him to slaughter a ram.
It has also been narrated through another route from Ibn Abbas (may Allah be pleased with them both) that he ordered a ram in a case like this.
The Shaykh (may Allah have mercy on him) said: The difference in his fatwas on this, and concerning one who vows to slaughter his son, indicates that he said it by inference and reasoning, not that he knew of an explicit ruling (tawqif) about it. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20085