(a) Abu Ubaydah narrates from Sayyiduna Abdullah (may Allah be pleased with him) that he saw a man who had placed his feet together in prayer, so he said: He has erred with regard to the Sunnah. Had he spaced them apart, it would have been more pleasing to me.
(b) It has been narrated to us on the authority of Abdullah bin al-Zubayr (may Allah be pleased with them both) that he placed his feet together in prayer.
(c) The report of Abdullah bin al-Zubayr (may Allah be pleased with them both) has already passed, that placing the feet together and placing one hand on the other in prayer is Sunnah.
The report of Ibn al-Zubayr is mawsul (with a connected chain), and the report of Abu Ubaydah from his father is mursal (with a broken chain). And Allah Almighty knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3570