Musnad Abu Ya'la al-Mawsili 110:271

Zuhayr narrated to us, Sa'id bin 'Amir narrated to us, from Hammam, from 'Ata', that Ibn al-Zubayr prayed the Maghrib prayer, then he performed the taslim after two rak'ahs. Then he stood up to touch the corner (the Black Stone), so the people glorified Allah (said "SubhanAllah") to him. He returned and prayed one rak'ah. He (the narrator) said: So I came to Ibn 'Abbas and informed him of that. He said: "He has not deviated from the Sunnah of his Prophet, may Allah bless him and grant him peace."