Sayyiduna Ibn al-Zubayr (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "A man swore by Allah, or swore falsely by Allah, and he was forgiven because of his sincerity towards Allah."
This is an error (wahm) on the part of Shu'bah; the correct version is the narration of the group. According to the historians, Ubaydah died nine years before Ibn al-Zubayr, so his narrating from him is unlikely, and Allah knows best. Ata bin al-Sa'ib alone narrated it, with disagreement over its chain from him. It was also narrated through Thabit from Anas, and that is not strong.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19877