Hadith 8:1341
Abu Bakr narrated to us, he said: Abu Usamah narrated to us, from Ubayd Allah ibn Umar, from Nafi, from Ibn Umar, who said: The Messenger of Allah (peace be upon him) prayed [two rak'ahs while traveling], and Abu Bakr [prayed] after him, and Umar [prayed] after Abu Bakr, and Uthman [prayed] for a portion of his caliphate. Then, Uthman later prayed four [rak'ahs]. So, when Ibn Umar prayed with the Imam, he would pray four, and when he prayed alone, he would pray them as two rak'ahs.
حدثنا أبو بكر قال حدثنا أبو أسامة عن عبيد الله بن عمر عن نافع عن بن عمر قال صلى رسول الله صلى الله عليه وسلم وأبو بكر بعده وعمر بعد أبي بكر وعثمان صدرا من خلافته ثم إن عثمان صلى بعد أربعا فكان بن عمر إذا صلى مع الإمام صلى أربعا وإذا صلى وحده صلاها ركعتين