Jeremy Corley argues that 1 Corinthians 12:31b-13:13 is original to the text of First Corinthians.
Jeremy Corley, "The Pauline Authorship of 1 Corinthians 13," The Catholic Biblical Quarterly 66, no. 2 (April 2004): 256-74
VI. Conclusion
This study has attempted to show that Pauline authorship of 1 Cor 12:31b-13:13 is indicated by several factors. First, there is no text-critical evidence that this chapter was ever missing from the epistle in early times. Moreover, the chapter abounds in Pauline vocabulary, as well as in expressions from the LXX and from the tradition of Jesus' teaching that Paul might well have known. many of the themes here occur elsewhere in the epistle or in other undisputed Pauline letters. The content of the chapter fits appropriately within the Apostle's argument in 1 Corinthians 12-14, while the chapter's style is suitable for a piece of epideictic rhetoric. Far from being ragged, the transitions at 12:31 and 14:1 point to a Pauline digression, similar, similar to those occurring elsewhere in the epistle. There are also echoes of 1 Corinthians 13 in an early apostolic father, Clement of Rome, and perhaps in Ignatius of Antioch. The cumulative value of this evidence suggests that, in accordance with the opinion of the majority of commentators, we have in 1 Cor 12:31b-13:13 a characteristic of Paul's writing.