Shevuot (Oaths) 20 - One Who Attaches His Oath To Another Oath
If one made an oath not to eat a certain loaf and then he said regarding another loaf, "This one should be to me like that one," - does this create another oath? Abaye says "Yes," but Rava says "No."
They challenged both based on the following ruling. If one says "It is hereby incumbent on me that I not eat meat nor drink wine, as on the day on which my father died (source for fasting on a yahrtzeit), or as on the day when Gedaliyah ben Achikam was killed (fast of the 3rd of Tishrei), or as on the day on which I viewed Jerusalem in its destruction" - he is indeed prohibited; and Shmuel commented on this, "This is only true if he already vowed from that day on."
This seems perfect according to Abaye: one attaches a vow to a vow, and in the same way an oath to an oath. However, it does not agree with Rava, who said that one cannot attach one oath to another oath! Rava will tell you that this ruling discusses a vow attached to a prohibited object (meat and wine), not an attachment to a vow, which, although mentioned, would be ineffective.
Art: Luis Meléndez - Still Life with Figs and Bread